Pair these roasted onions with grilled steak or shrimp for a restaurant-worthy dinner.
Preheat oven to 425°F. In 3-quart rectangular baking dish combine butter and olive oil. Whisk in vinegar, wine, sugar, salt, and pepper. Set aside.
Peel off papery outer layers of onions, but do not cut off either end. Cut onions in half from stem through root end. Place onions in dish, cut sides up. Cover loosely with foil and bake for 30 minutes.
Remove foil. Using tongs, carefully turn onions over to cut sides down. Bake, uncovered, for 20 to 25 minutes longer or until onions are tender and balsamic mixture is thickened and caramelized. Serve cut sides up. If desired, sprinkle with fresh thyme.
Prepare onions as directed. Cool, cover, and chill for up to 24 hours. To reheat, preheat oven to 325°F. Bake, covered, about 40 minutes or until heated through.