Preheat the oven to 375 degrees.
Cut the onions in half and slice 1/4 inch thick. Place on a baking sheet and toss with 1/4 cup balsamic vinegar, the olive oil, salt, and pepper. Bake for 10 to 12 minutes, until the onions are tender. Remove from the oven, toss with 2 more tablespoons balsamic vinegar.