1. Leaving root end attached, slice onions in half lengthwise. Peel papery skin off onion. In a 9- by 12-inch baking pan, mix garlic with 2 tablespoons of the vinegar, oil, salt, and pepper to make a marinade. Add onions. Toss marinade with onions. Let sit for 1 hour.

2. Preheat oven to 350 degrees F. Bake onions in marinade for 1-1/4 hours. Onions are done when a knife inserted into center meets no resistance. Remove from oven to a serving dish and sprinkle with remaining vinegar.