Preheat oven to 425°.
Remove roots and outer leaves from leek, leaving 1 inch of green leaves. Discard remaining leaves; rinse with cold water. Remove 1 inch green leaves; cut lengthwise into 1-inch strips. Set aside. Cut white portion into thin slices to measure 1/2 cup.
Combine white portion of leek and potato in a 13 x 9-inch baking dish coated with cooking spray. Drizzle with 1 tablespoon oil; toss well. Place green portion of leek in an 8-inch baking dish. Drizzle with 1 1/2 teaspoons oil; toss well, and set aside.
Bake potato mixture at 425° for 20 minutes, stirring once. (Do not remove from oven.) Add baking dish with green leek tops to oven. Bake at 425° for 15 minutes or until potato mixture and green leek tops are tender, stirring both after 8 minutes. Remove baking dishes from oven.
Place milk in a large microwave-safe bowl. Microwave at high 2 minutes or until warm. Add potato mixture; beat at medium speed of a mixer until well-blended. Stir in salt, pepper, and sour cream. Sprinkle with green leek tops.