Bring potatoes, 1 teaspoon salt, and water to cover to a boil in a Dutch oven; boil 20 to 25 minutes or until potatoes are tender. Drain, and keep warm.
Remove root, tough outer leaves, and tops from leeks, leaving 2 inches of dark leaves. Thinly slice leeks; rinse well, and drain.
Melt 2 tablespoons butter in a large skillet over medium-high heat; add leeks, and saute 5 minutes. (Do not brown.)
Mash potatoes with a potato masher; stir in remaining 1/2 teaspoon salt, 1/2 cup butter, and next 3 ingredients until blended. Stir in leeks.