From Gourmet, http://www.epicurious.com

Serves 6 to 8.

3 1/2 cups finely chopped white and pale green part of leek, washed well and drained
3/4 stick (6 tablespoons) unsalted butter
3 pounds russet (baking) potatoes (about 6 large)
6 garlic cloves, unpeeled
1/2 cup of milk

In a large heavy skillet cook the leeks and 4 tablespoons of the butter, covered, over moderate heat, stirring occasionally, for 8 to 10 minutes, or until they are very tender, and keep them warm. In a kettle combine the potatoes, peeled and cut into 1-inch pieces, the garlic, and enough water to cover the potatoes by 1 inch, bring the water to a boil, and simmer the potatoes for 10 to 15 minutes, or until they are tender. Drain the potatoes and garlic in a large colander, return the potatoes to the kettle, reserving the garlic, and cook them over moderate heat, shaking the kettle, for 30 seconds to let any excess liquid evaporate. Peel the reserved garlic, force the garlic and the potatoes through a ricer or the medium disk of a food mill into a bowl, and stir in the remaining 2 tablespoons butter, the leeks, the milk, and the salt and pepper to taste.