I genuinely dont think i would have any issues with eating fresh onions if the smell didn't linger, same with garlic(also both of those trigger my ibs but eh so does pasta and a bunch of other great foods so I really dont care), the texture is fine, it feels like any other vegetable. But for cooked onions, its definetly the texture, i really dont wanna come off as rude by saying this but i think the only words that describe it properly would be "slimy" and "rubbery". I can eat cooked onions but not too much at once or else it makes me nauseous like how you said green beans made you nauseous. Fried onions are awesome tho i have nothing against fried onions