Substitute for onions

My mom's digestion has changed over time and now onions tend to upset her digestion.
We do a lot of savoury cooking (roasts, pastas, chili, meat pies, etc) and nearly everything utilizes onions for their dynamic flavours, but we rarely can (we occasionally get to just use a hint, but mom usually still pays for it). And we can often use garlic to a degree, especially based on if we are working that day/the next or not.
Sometimes we can use leeks, as they are not as harsh but it doesn't seem to lend itself to every instance.

Any suggestions? Even if it is a different flavour? Basically I am looking for some general suggestions that will add flavour to a dish, instead of leaving things bland.
(And if you have some suggestions for a similar tast to the onion that would be great, because I'm running low on ideas.

Thanks