It's an ADHD texture thing, the texture of onions does not mesh with any other food (maybe edible paper?), so it's always jarring to bite an onion while eating food.
I'm with you when the onions are raw. Cooked, the texture is tolerable enough that the taste is worth it IMO. Though I do prefer them chopped small or sliced thin no matter what.
My brother is the same but he has always been „problematic“ with all greens. He at least tried it and I think had a bad experience due to wrong cooking, so now he has a trauma of some kind, I don’t know. Onions can have so many different textures depending on if or how they are cooked. If it doesn’t fit with the rest of the food, it was probably prepared wrong.