Preheat the oven to 375 degrees F.
Coat the bottom and sides of an 8 by 8-inch baking dish with 1 tablespoon butter. Set aside.
In a large pot, add enough water to fill 3/4 of the way, and then add the potatoes, garlic and a hefty pinch of salt. Bring to a boil and cook until a fork is easily inserted, but the potatoes still hold their form, about 20 minutes. Strain in a colander and place back over the empty pot, allowing the remaining heat to evaporate some of the moisture from the potatoes and garlic, just a few minutes. Mash the contents of the pot well, and then add the remaining 2 tablespoons butter and the heavy cream. Mash until smooth, but if you like it chunky, leave it chunky. Then stir in 1 cup pepper jack cheese and 1/4 cup Parmesan. Spoon into the prepared baking dish and spread evenly. Stir together the remaining 1/4 cup Parmesan, 2 tablespoons jack cheese and breadcrumbs. Sprinkle the mixture evenly over the top of the potatoes. Bake in the oven until the top is golden brown, about 20 minutes. Serve warm.
Recipe courtesy of Sunny Anderson, 2010