How to Make It
Preheat the oven to 350°. Stand the garlic on a large sheet of heavy-duty foil. Drizzle with oil and wrap in the foil. Bake for about 1 hour, until the garlic is very soft. Squeeze the soft cloves from the skins into a small bowl and mash with a fork.
Meanwhile, bring the potatoes to a boil in a large pot of water. Salt the water and boil over moderately high heat until tender, about 20 minutes. Drain and return to the pot. Shake the pot over moderately high heat for 1 minute to dry the potatoes.
Mash the potatoes with a potato masher, then mash in half of the hot cream. Add the remaining cream and mash again. Stir in the Manchego to melt, then slowly stir in the mashed garlic and 3 tablespoons of the olive oil. Season with salt and transfer to a serving bowl. Drizzle with the remaining 1 tablespoon of olive oil and serve.