Cut a 1/4-inch slice from top and bottom of each peeled onion. Reserve slices for use in another recipe. Partially core each onion.
Place onions in a lightly greased 13- x 9- x 2-inch baking dish. Place 2 teaspoons butter in each onion. Cover dish tightly with aluminum foil, and bake at 400° for 1 hour. Uncover, and bake an additional 5 minutes or until tender. Sprinkle with salt and white pepper to taste.