For the cheese sauce, heat the butter in a saucepan over medium heat. Once it is melted, add the garlic and cook for 15 seconds, then whisk in the flour to create a roux. Stir the roux until it is golden in color, 1 to 2 minutes. Pour the milk in slowly and whisk constantly until the liquid starts to thicken, another 5 minutes. Reduce the heat to low and add the mascarpone, Gruyere, 6 ounces of the cheddar, 6 ounces of the Fontina and the Parmesan cheese. Stir the sauce constantly with a heatproof spatula until the cheese is melted, then add the pepper, salt, and nutmeg. Stir to combine.