Then there are burgers and sandwiches, where bits of raw onion get hidden in the middle, only to be discovered after a horrifying crunch with a stinging on my tongue.

I enjoy that. I was gonna say raw onion slices go hard in a burger. But I get what you're about, as there was a kebab place in my city decades ago which put fkin whole cloves of garlic in the roll, not crushed or anything. And that was too much for me. Crunch in them and get all the bitterness eww. Maybe they had been pickled a bit idk but mostly raw. But I'm clearly not as sensitive. I enjoy the crunch in a burger like you say, although I think it maybe contribute to the taste as well.