Preheat oven to 400°.
Combine first 6 ingredients in a small saucepan; bring to a boil. Reduce heat to medium; cook, uncovered, until reduced to 1/2 cup (about 10 minutes). Set aside.
Cut the onions lengthwise in half. Place the onions, cut sides down, in a small baking dish coated with cooking spray. Drizzle olive oil over onions. Cover onions and bake at 400° for 25 minutes. Uncover and bake 20 minutes. Remove onions from oven. Turn the onions over, and pour the wine mixture over the onions. Bake for an additional 20 minutes or until the onions are tender, basting every 5 minutes. Serve onions with wine mixture.