In a 12- by 17-inch pan, combine vinegar, 1 1/4 cups water, brown sugar, salt, and pepper.
Cut onions in half lengthwise. Lay onions cut side down in pan.
Bake in a 400° oven until onions give readily when they are gently squeezed and most of the liquid has evaporated, 50 to 60 minutes. If liquid cooks away, add a little water to prevent scorching. Arrange onions cut side up on a platter and pour pan juices over them. Serve hot or at room temperature. Add salt to taste.