Place potatoes in a Dutch oven, and cover with water. Bring to a boil; reduce heat, and simmer for 20 minutes or until tender. Drain. Cover and chill.
Preheat oven to 350°.
Peel and shred the potatoes. Place in a 13 x 9-inch baking dish coated with cooking spray. Melt butter in a large saucepan over medium heat. Stir in the milk, 1/2 cup cheese, salt, and pepper. Cook 8 minutes or until cheese melts, stirring occasionally. Pour over potatoes, and stir gently. Bake at 350° for 45 minutes. Sprinkle with 1/2 cup cheese. Bake an additional 15 minutes or until cheese melts.