Onions are an ingredient in almost every recipe. I hate the taste(flavour) of them (I know I might be the only one). I always try to avoid eating them and try to pick them out of the food. Seeing that I am the only one in the house that doesn’t like the taste of onions not using them in a recipe would be selfish.
I was just wondering (a) if there would be a good substitute or (b) do I really need to have them in a recipe? And (c) what purpose do they add to food?
As a child, I used to hate the feel and taste when I conciously bit on a piece of onion (strongest offender: medium-sized chunks, sautéed glassy but still firm). Sometimes that still happens.
However, I use onion a lot myself now in cooking. Lots of dishes really need the kind of taste onions can supply; even though that taste may by itself be a bit gross, it's really useful in the mix. (This is true for many other ingredients as well!) I can only speak for myself, but very likely, there are some dishes where you don't even notice that onions are in, but they are – and you wouldn't like it if they weren't!
Do use onions, just do it consciously. There are broadly three ways of preparing them that I consider effective, each suited to different cases:
Perhaps you won't be able to befriend yourself with all of these preparation methods, but you should give them a try.
Their primary purpose is flavor, although (as mentioned by @David Richerby) they can also be used for thickening in some cases. If you don't like them, but other members of your household do, then it becomes a cost-benefit analysis of every dish.
If your hatred of onions is visceral, yet other family members would just kind of rather have them than not, not including them is easy. It becomes trickier if everyone else loves them.
Consider prepping onions by quickly soaking the chopped onions in cold water or giving them a quick pickle in vinegar and salt. That may remove flavors that you find offensive while still keeping those qualities that other members of your household may like.
Garlic provides much of the same flavor profile as onions. Can you add a bit more garlic when you eliminate onions?
You might find some types of onions less offensive than others. Consider supersweet onions like Vidalia, green onions (AKA scallions), and red onions that have been tamed a bit by soaking or pickling.
Consider asafoetida as a substitute, along with other substitutions found here: What can I substitute for onions?
As a onion-lover I suggest using a similar vegetable instead of onions themselves. Try with leek, it has a lighter flavour than onion. Cook it in the same way.
Another option would be shallot, which has a stronger presence but is different.
In some soups or salads, also dried onion is a viable option, as you can add it directly on the servings.
I would avoid garlic, as it is very different from onion and many people dislike it.
Try onion powder. It has much of the flavor but won't give you slimy chunks of something you don't like.
(Just a warning, it's a gateway drug and pretty soon you will be using garlic powder, then garlic cloves, then shallots and finally real onions chasing that first high).
People are disagreeing, but I hated onions too, taste and texture, and found onion powder to be a "softer" version of both. It's really meaningfully different in a way that's worth trying, and it's shelf stable, cheap and easy. Not fancy by any stretch, shallots will be better for finer dining, but day-to-day I use onion powder all the time.
Go make a sandwich with onion powder, salt, pepper, paprika and mustard mixed into the mayo, then come back and tell me it's not better than plain! Another quick test is a bean burrito with onion powder, chili powder, paprika and salt mixed in the beans.
Asafoetida, also known as hing powder, is a good option as a substitute for alliums like onions and garlic (also as a complement to!). It is used in a lot of Indian cooking; big in Hare Krishna cooking .
It comes as a powder, which is from the ground resinous sap of a particular root. You can pick it up in most Asian / Indian grocery stores (in Australia, at least).
When I use it, I briefly fry it (e.g. 15 seconds or so - til fragrant) before adding bulkier, more liquid ingredients (so somewhat like garlic). I've found this brings out the flavour; I found it a bit unpleasant if not fried beforehand, but it really is quite special if you treat it well!
Also (as per good advice in comment below), it is best to store it well sealed in a glass jar; the scent can be quite strong if it isn't properly stored, and some people aren't so keen on it (hence the etymology of 'smelly resin').
People use onions for the flavor. You may have noticed that not only do many recipes include onions, but there often a few other ingredients as well at the start of the recipe - specifically the addition of carrots and celery, or carrots and green pepper. These ingredients appear in many recipes because they offer a balanced flavor to build other recipes on:
This article has some good info about Mirepoix and worldwide variations of this flavor base across a few countries. http://www.seriouseats.com/2014/05/all-about-mirepoix.html
If you hadn't noticed, all the countries listed in the article are European or had strong European influence. So if you don't like this trio of flavor, you will need to look outside of European style food. I can already think of a few Middle Eastern/Mediterranean recipes that use garlic, but no onion. Korean recipes that use garlic, and only green onions. Japanese food doesn't really use onion or garlic, and Ethiopian food has some but not too much onion.
Some recipes may have onions that have been stewed so long (2+ hours) they've become mushy and don't taste like onion at all. You could also try submerging them in a brine to change the flavor. You might have luck there.
Lastly, you might try changing the type of onion you use. There are many different varieties of onions, from leeks to spring onions, to cippolinis to ramps. If you don't want to skip a recipe entirely, maybe you can use a milder onion instead, so it's a less intense flavor than what you'd get from a white or yellow onion. (You could also try soaking your onions, or try to modify the flavor by putting them in some kind of brine or pickling liquid. Pickling liquids can be based on salt or vinegar, and can be sour or sweet. That would obviously change the final taste of your recipe.)
I don't know if there's a way to use a recipe that calls for onions and not feel that there's a flavor missing. I don't think there's a substitute, except using another type of onion/garlic.
For onions, I'm with some of the folks above - I have been presented with them in ways that I find quite literally disgusting, but cooked other ways they are fine.
However, I feel about peppers, all peppers (excepting the "peppercorns" that make the ground black or white pepper on the table) the way you claim to feel about onions. As such, I'd recommend not using them at all, at least in anything that you are eating. If you feel that you need to cook dishes for your family that have them, cook a separate dish for yourself that does not, or cook the onions separately and add them to the dish after removing part of the dish for yourself. Once peppers (very much including green bell peppers) have contaminated a dish, it's useless to me - removing the source of the foul taste does not remove the foul taste throughout the dish.
For your three point question:
A) Nothing else is really a direct substitute, and I suspect that most will be too close if you feel this strongly about onions; ie, I'd be surprised if you like garlic, leeks, or shallots, all close relatives of onions.
B)You can leave them out if you are an adaptable cook. If you are inflexible and following a recipe that depends on the liquid in the onions, you might have issues. If you see that things are too dry and compensate, should be fine, other than...
C) It's for the (highly variable, depending how they are prepared) taste. But if you don't like that, it's not a good reason to use them, is it? If, however, you have an objection to one part of the flavor/texture spectrum that they provide, but not others, preparation/cooking becomes key. But I know that there is no method of preparing or cooking a pepper (other than throwing it in the trash, not in the food) that makes it taste good to me; and if that is your relationship to onions, skipping them is your best bet.
(a) if there would be a good substitute
If you strongly don't like onions and are inflexible in that regard, You will not find a good substitute. You like what you like and don't like what you don't like, substitutions can't change that.
Though, if you want to try a substitute: try dehydrated minced onions (usually found in the spice section of the grocer). Aspects:
I use dehydrated onions with one of my children who hates onions. She actually welcomes the dehydrated onions.
(b) do I really need to have them in a recipe?
Usually: No, you do not need them. Drop them from the recipes, come back here with any specific problems that may cause.
(c) what purpose do they add to food?
In the dishes I cook, they are there for flavor. Others have mentioned emulsifying and other factors, but I believe the majority of onion entries in recipes is for flavor, and that onion may be omitted in those cases without consequence.
Green onions are a great alternative that are much less intense in flavor, have a different texture, usually make your dishes look nicer, and essentially fill the same role.
Cut perpendicular to the stem on to get round rings or at a 45° for longer tubes.
Onions are a good edible emulsifier, as is garlic, turning the fats and juices from any meat into a rich, water-soluble gravy.
Really been cooking for more then 2 decades and never liked onions since I'm a kid (not allergic though) so none of them got into my house or kitchen as a adult. I've been learning to try any dish whether containing onions or not but I still don't eat onions regularly or incorporate them in my diet (garlic's in though for other reasons) in any case and also been super-lucky to find a significant other with the same dislike for onions, so no onions in my cooking to this day (over 25 years of cooking). In essence you don't need them, but always be open minded, they are used for flavour and in some recipes for thickening but those can be easily replaced by leeks, chives or (relatively tasteless) thickening agents such as crushed flax seeds or eggs. At the end of the day it's up to you, I've been lucky to spend all of my adult life onion-free and sucka-free, it's up to you and the people you love (and love you enough to compromise).
Onions were used as a way to collect bacteria as it passed through the gut, when using less than perfect meat. It was a way of preventing people from getting sick before Refridgeration. This is one of the reasons it is included in stews so often. Eating an onion whilst sick with food poisoning will rapidly reduce the period of time with symptoms. The taste aspect then in modern cooking can be eliminated if it is too strong.
