With a small, sharp knife, cut top and bottom off 3 oranges (10 oz. each), cutting deeply enough to reveal flesh. Set orange, with one cut end down, on board. Slice down sides of oranges to remove white pith and to reveal flesh; discard peels. Thinly slice orange flesh crosswise into rounds. Arrange orange slices on a platter. Peel 1 white onion (5 oz.); cut half into thin rounds. Separate rounds into rings and rinse and drain; distribute evenly over oranges. (Reserve remaining onion half for another use.)
Mix 2 tablespoons white wine vinegar and 1 tablespoon extra-virgin olive oil.
Drizzle evenly over onions; sprinkle with salt and fresh-ground pepper to taste.
