What can I substitute for onions?

Leeks can provide some of the same flavors as onions, but the flavors are lower intensity than onions.

With the exception of dishes with "onion" in their names, more often than not you can omit onion as an ingredient. My advice would be to first try leaving the onion out entirely, see how the dish is. If it is missing something, then experiment with:

1. Onion powder: see how much you can get away with before your audience notices.
2. Garlic: add more to compensate.
3. Shallots and other onion-like vegetation: give it a try to see how that goes over.

It is going to require some experiment and trial & (lots of) error on your part, but you should be able to strike a nice balance in a few attempts.

In India, some Brahmin caste cooks and Jains aren't allowed to use onions or garlic in their cooking, and so they substitute Asafoetida spice instead.

I can't stand onions due to texture issues unless very finely chopped, but I find that shallots have similar effects on the flavor of many foods.

In addition to the other suggestions such as shallots, garlic, or green onions, another avenue to explore would be using a sweet yellow onion instead of a white onion or purple onion. Their flavor is a bit milder and may be more palatable compared to a white onion, though if texture is the primary problem, grinding the onion up in a food processor until it is finely chopped may be a better choice.

You may cut the stem of some vegetables in thin slice (e.g. reddish, Chinese vegetables with long rod-like stem). They feel like onion when you chew them but taste like normal green.