Preheat the oven to 400. Separate the head of garlic into cloves and peel. Scatter the cloves and the onion slices on the bottom of a baking dish that will hold the potatoes in a single layer. Place the halved potateos, cut side down, on the bed of garlic and onions. Pour in enough stock to come about one quarter of the way up the sides of the dish. Sprinkle salt and pepper evenly over all, then dot with butter. Bake, uncovered, for one hour, or until the potatoes are tender, the onions beginning to brown and the liquid about gone. Serve piping hot.