Roasted onions, sweet and slightly caramelized, are always a favorite vegetable side dish.
Preheat oven to 450 degrees. On a large rimmed baking sheet, toss onions with extra-virgin olive oil and 3 small sprigs rosemary. Arrange onions, cut side down. Roast, without turning, until onions are browned on underside, about 15 minutes. Flip and roast until onions are tender, about 5 minutes more. Transfer onions to a serving dish and drizzle with white-wine vinegar.