Put garlic and olive oil in heavy saucepan over lowest possible heat and simmer until soft; 30 to 40 minutes. Drain off oil (reserve for marinades or vinaigrettes). Puree garlic; set aside. Meanwhile, prick potatoes with fork and bake in 400° oven for 1 hour, or until soft. While still hot, peel and mash, or pass through a potato ricer. Melt butter in heavy cream; whisk in puréed garlic. Stir into potatoes. Stir in cheeses and season with salt and pepper. Spoon into a gratin dish. Place in a 400° oven for 12 to 15 minutes or until browned and bubbling.
8 to 10 cloves garlic, peeled