8 teaspoons balsamic vinegar

Preheat oven to 350 degrees. Butter a baking dish just large enough to hold the onions. Cut the unpeeled onions in half lengthwise through the root and stem ends. Cut a thin slice off the rounded side of each half so that it will sit upright in the baking dish. Arrange the onion halves, cut side up, in the prepared baking dish. Season with salt and pepper, and place 1 teaspoon of butter on each half.

Cover and bake 45 to 50 minutes or until onions give slightly when the edges are squeezed together between your fingers. Uncover and sprinkle each onion half with 2 teaspoons of balsamic vinegar (use a fork to spread the onion layers apart so that the vinegar can dribble down between the layers). Bake, uncovered, another 15 minutes. Remove from oven and serve hot or at room temperature.