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Almost every soup recipe seems to call for sauteed onions. For instance, I'm looking up recipes for squash soup, and every single one calls for onions -- but every soup recipe I can think of uses onions, so it's not just this type.

So what's the purpose of adding onion?

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up vote 15 down vote accepted

Sauteed onions can provide both caramel flavors (from the sugars in the onions) and Maillard reaction compounds, depending on how they are sauteed. Thus onions can supply a range of "umami" flavors for soup which otherwise you need to get through roasting animal bones and other tissue (e.g. brown veal stock). Of course, even beef stocks often add onion as well for extra flavor.

As an extreme example of this, I often prepare a vegetarian French Onion soup using a meat-free broth made entirely from onions, onion skins, and cheese rind. Blind tasters often fail to distinguish it from a store-bought beef stock.

You can get a lot of the same flavors from combinations of other browned vegetables, but onions neatly supply a perfect package of flavor compounds in one inexpensive, long-keeping root vegetable. Why use anything else?

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Thanks, that makes sense. – The How-To Geek Dec 20 '11 at 17:33

Onion is a great way to add moisture without adding liquid. It Carmelizes beautifully and has a unique sugar to add that makes the palate water and t thereby stimulate chemical breakdown and taste receptor activity. As a component of mire-poix it is a traditional component of older soups (typically of European origin). The chemicals that make your eyes water open up your gustatory system to a broader spectrum of flavor.

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It is common in many cultures' version of Mire-poix. The Trinity in Creole/Cajun cooking (Onion/Celery/Bell-pepper) is an example. Here are some others. The onion's flavors add a nice, savory complement to many soups, sauces, and entrees. It is also relatively cheap wherever you go, so it makes sense it would be used around the world to make 'peasant food' more palatable. – JSM Aug 15 '14 at 21:55

It's a good flavor that goes with just about anything. I think I use onions in about 80% of savory dishes that I cook.

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So every soup needs to taste like onion? I was assuming there was something else to it. – The How-To Geek Dec 18 '11 at 1:20
Yes, it is the flavor. Just like almost every steak recipe says you have to season with black pepper. It is partly tradition, partly convenience (easily grown in all of Europe, easily stored, cheap, keeps well in the winter, so always available), and partly a great taste fit for most soups. But there are definitely good soups which don't include onion. – rumtscho Dec 18 '11 at 16:29

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