Peel the potatoes, quarter them lengthwise, and cut them into 1-inch pieces. Reserving 3 of the garlic cloves for the ragout, in a steamer set over boiling water steam the remaining garlic cloves with the potatoes, covered, for 12 to 15 minutes, or until the potatoes are very tender. Force the steamed garlic and the potatoes through a ricer or food mill fitted with the medium disk into a large bowl, stir in the butter, the milk, and salt and pepper to taste, and keep the purée warm, covered. The purée may be made 1 day in advance, kept covered and chilled, and reheated.