Raw red onions can give you an unpleasant shock. They are supposed to be mild, and sweet; but often they are not.
A good way to ensure sweetness is to slice the onions, put them in a bowl, and cover them with boiling water. Leave them for half an hour, or longer, and drain them, squeezing out the liquid (with your hands, or with the back of a spoon pushed gently against the onions in a sieve). The milkiness in the bowl, and in the liquid that emerges from the squeezed onions, is sulphur, which is what causes raw onions to burn our mouths and sting our eyes.
I like red onions in salads with beans and/or rice. It might go, instead of the fried onions, in this recipe, for example.