Fill a large stockpot with water and bring to a boil over high heat. Add a generous amount of salt, return to a boil, then add the potatoes (the water should cover the potatoes by at least 2 inches). Bring to a simmer, reduce the heat to medium and continue to simmer for 15 or so minutes, partially covered, until the potatoes are very tender when pierced with a knife.
Drain the potatoes and return them to the pot, and place them back over medium-low heat. Toss the potatoes in the hot pan occasionally for 3 minutes or so until the moisture is all gone and they have begun to dry out (but not to brown). Remove the pan from the stove, and put the potatoes through a ricer or a food mill, or mash with a potato masher until they are smooth, or as smooth as you like them. Return them to the pot. Meanwhile, heat the milk and the cream together in a microwave safe bowl or pitcher for about one minute, or in a small saucepan on the stovetop, until hot.
Add the hot milk and cream mixture to the potatoes along with the butter and stir with a wooden spoon or a whisk until well combined. Blend in the Fontina and the Grand Cru, adding the cheeses by the handful, and stirring after each addition until the cheese melts into the potatoes. Add salt and black pepper to taste, and stir over medium low heat until everything is hot and well blended. Serve hot.
This post has been sponsored by Roth cheese. All the opinions expressed are my very own.