Preheat oven to 350 degrees F. Butter a baking dish just large enough to hold the onions.
Slice of the top and the bottom parts of the onions so they sit flat in the baking dish; cut the onions in half.
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 onion half.
Cover and bake 50 to 60 minutes or until onions give slightly when the edges are squeezed together between your fingers. Remove from oven. Uncover and sprinkle each onion half with balsamic vinegar (use a fork to spread the onion layers apart so that the vinegar can dribble down between the layers).
Serve hot or at room temperature.
Makes 4 servings.
* Sweet onions have a thinner, lighter color skin than storage onions and tend to be more fragile. Signs in produce sections usually differentiate between sweet onions and storage onions. Look for sweet onions that are light golden-brown in color, with a shiny tissue-thin skin and firm, tight, dry necks. Avoid onions that are soft or sprouting. They should have absolutely NO SMELL whatever. If they do, they are probably bruised somewhere under the skin and are on their way out.
** This is the grade of balsamic vinegar used in your cooking, such as for a flavorful salad dressing, to flavor enhance soups and stews, and also used as a marinade.
Roasted Balsamic Sweet Onion Recipe: https://whatscookingamerica.net/balonion.htm