PREHEAT OVEN TO 220ºC . Combine butter, potatoes, garlic, onion, salt and pepper in a bowl and mix well. Place in a baking dish, cover and place in oven for 10 minutes. Scald the milk in a small pot on the stove, then add it to the potatoes, and cook, uncovered, another 20 minutes. Add the grated cheese and the egg to the potatoes in the baking dish and mix well. Replace in the oven until the top is browned, another 10 minutes or so.