Place potatoes in a large saucepan. Cover with water. Bring to a boil and cook until potatoes until tender, about 15 minutes. Remove potatoes from pan to cool. Peel potatoes and cut into pieces. Mash with a potato masher.
Mix in 1 tablespoon butter, salt, pepper, garlic, nutmeg and 1/2 cup Parmesan. Mix well.
Preheat broiler. Butter a shallow 2-quart baking dish.
Spread potatoes in pan and sprinkle with 1/4 cup Parmesan. Dot with remaining 1 tablespoon butter.