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.