I don't like onions very much so I often use extra garlic instead of onions in a recipe, I wonder if it could work the other way also? Maybe add a little (extra) onion or some shallots?

I am allergic to both onion and garlic (paprika and mustard too) I typically will use red pepper flakes for a spicier taste or use parsley or cilantro for a brighter taste. Lemon is also used often.

What about shallots?

I find red onions much easier to digest.

If they are allergic to garlic they may be sensitive or allergic to other foods in the allium family such as shallots and onions. Depending on the recipe, ginger could be a good alternative or peppers.