Place potatoes in a large saucepan, add cold water just to cover and 1 tablespoon of salt. Bring to a boil over high heat and cook until tender. Drain well and run through a food mill set over a large bowl.
While the potatoes are cooking, combine the milk, garlic puree and butter in a small saucepan and bring to a simmer over low heat. Stir the milk mixture into the potatoes until combined. Fold in the mascarpone and season well with salt and pepper. Keep warm over in a bain marie or double boiler until serving.