Put the garlic and olive oil in a heavy saucepan over lowest possible heat and simmer until soft; 30 to 40 minutes. Drain off oil. Puree garlic; set aside.

Meanwhile, prick potatoes with a fork and bake in a 400°F 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 pureed garlic. Stir into potatoes. Stir in cheeses and season with salt and pepper. Spoon into a gratin dish. Place in a 400°F oven for 12 to 15 minutes or until browned and bubbling.