cut into 3 inch pieces
Bring a large pot of water to a boil. Add the potatoes, garlic, and salt and simmer, partially covered, over medium heat for 15 to 20 minutes, or until the potatoes and garlic are fork-tender. Drain the potatoes and garlic and return to the pot over high heat. Cook, tossing the potatoes, for 1 to 2 minutes, or until all the moisture is evaporated. Transfer to a large bowl and mash with a potato masher.
In a medium saucepan, heat the butter, oil, and milk over medium heat until the butter has melted. Add to the potatoes and whip with a wooden spoon or whisk to a smooth but not soupy consistency. Mash any pieces of garlic until totally pureed. When the liquid is absorbed, season with salt and pepper to taste. Transfer to a serving bowl and serve immediately. To prepare in advance, make up to 4 hours ahead, cover, and keep at room temperature. Reheat gently in the top of a double boiler over medi