I have a lamb stew recipe which I'm trying to adapt for my sister, who can't eat onions, garlic, or other alliums. Usually when I need to substitute for garlic or onion, I use a tiny amount of asafoetida, which works well enough for that strong alium taste. However, that's not the role of the onion in this stew, and I'm a little stumped for how to substitute it out.
For starters, this stew only has three ingredients: lamb stew meat, onion, and water (plus a host of spices). It simmers for about 2-3 hours, during which point the onion dissolves completely and the tough meat becomes tender. (It's then served over rice; not important to the narrative.)
The onion serves to add a touch of sweetness, which mellows out the lamb meat. It also provides a slight thickness to the liquid. I tried omitting the onion entirely, and the end result was nearly harsh -- it's definitely something I want to try and substitute out instead of simply omit.
Asafoetida is a decent substitute for the garlicy/oniony taste, but that's not really the primary characteristic I'm going for here -- I'm looking for that mild sweetness that shows up after you cook onions low and slow until they dissolve into nothing. For thickness, I figure I can blend in minute quantities of corn starch until I reach the desired consistency -- but I can't figure out what to use for the sweetness.