Cut top of garlic head off and discard. Place garlic on a square of foil, drizzle with Olive oil and bring up all 4 corners into a package. Place in a 400 degree oven for 1 hour. Remove and open foil to cool.
Place thawed potatoes in a large mixing bowl, add melted butter, onion and garlic by squeezing the garlic head directly over potatoes and discarding the papery garlic shell. Mix.
Gently mix in grated cheese and turn into a sprayed baking dish. Bake at 350 for 45 minutes or until thoroughly heated and golden on top. Serves 4-6. Great leftover!