Place ducks, breast side up, on a rack in a large roasting pan, folding neck skin under. Combine salt, pepper, and flour; stir well. Sprinkle ducks with flour mixture. Combine water, butter, and onion; pour into roasting pan. Bake, uncovered, at 400° for 1 hour or until drumsticks and thighs move easily. Garnish with lemon slices, if desired. Serve with Orange Sauce.