Place halves of squash, cut side down, in a shallow baking dish. Add 1 inch of water. Bake, uncovered, at 400° for 30 minutes or until tender.
Remove squash from baking dish, and cut into quarters. Place in a 13- x 9- x 2-inch baking dish.
Combine butter and orange juice; pour over squash. Sprinkle sugar over squash. Return to oven; bake 10 minutes. Garnish with orange slices, if desired.