Cut an onion in half lengthwise. Cut each half into 1/4 inch slices. Toss with about a teaspoon of olive oil, a teaspoon of balsamic vinegar and salt and pepper to taste. Roast at 375 degrees for about 15 - 20 minutes, stirring occasionally. After the onions are tender, remove from oven and toss with another teaspoon of balsamic vinegar. You can easily double or triple this recipe to make as many as you need.
These are great to make in advance. We had them tonight on a bed of arugula with gorgonzola and dried cranberries.