Onions are one of those foods that make me retch immediately upon biting into them and I have to spit them out. See also garlic, cauliflower, broccoli, and cabbage (with the last three I can't even bear the smell of them cooking. So nasty.)
Btw I like McDonald's. It's become cool to hate them but their fries and milkshakes are about as good as they get in fast food.