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. * 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.