This is a salad I grew up with. It is wonderful as a garnish for churrasco. If you think you can't possibly eat raw onions, the secret is to soak the sliced onions in ice water or sprinkle them with sugar and leave them in the refrigerator for a couple of hours. That will take away some of the pungency. Then when you eat them, cut the rings into smaller pieces with your fork and knife..
2 large onions
Peel the onions and slice them into very thin slices. Separate the slices into rings. Place the onions in a bowl and cover them with ice cubes and water. Refrigerate for 2 hours. The alternative used by my friends in Bahia is to sprinkle the slices with sugar (2 Tbsp) and refrigerate for 2 hours. Both methods have worked for me.
When you are ready to serve them, drain the onion slices and rinse under cold water. Serve them in a glass bowl, drizzled with extra-virgin olive oil and red wine vinegar, and sprinkled with salt and freshly ground pepper.