In a 5-quart saucepan, combine the potatoes, garlic, 1 teaspoon of salt, butter, and enough cold water to cover. Bring to a boil over medium-high heat. Reduce the heat to a simmer and cook until the potatoes are tender, about 15 to 20 minutes. Remove the garlic. Drain in a colander, reserving 1/2 cup of the cooking liquid. Return the potatoes to the pan. Coarsely mash the potatoes with a fork adding enough cooking liquid to moisten, 1/4 cup at a time. Stir in the Parmesan cheese, oil, and mascarpone cheese. Season with the remaining salt and pepper.