Step 1Heat the oven to 325 degrees. Butter an 8-by-12-inch square or oval gratin pan. Cut the garlic in half and rub the cut side all over the interior of the pan. Set aside.
Step 2Combine the Gorgonzola with the fontina and one-fourth cup Parmigiano-Reggiano. Set aside.
Step 3Peel the potatoes and place them in cold water. Remove 2 potatoes and pat dry. Thinly slice them and overlap the slices in the gratin dish. Season well with salt and pepper to taste. Distribute the combined cheeses evenly over the potatoes. Thinly slice the remaining potatoes and layer them over the cheese. Season with salt, pepper and nutmeg.
Step 4Pour the cream evenly over the potatoes. Distribute the remaining Parmigiano-Reggiano evenly over the cream.
Step 5Bake 1 hour, 15 minutes, until the cream is bubbly, the top is well browned and the potatoes are tender. Let stand for 5 minutes before cutting into squares to serve.