Place potatoes in medium saucepan and cover with water. Cook until soft but not falling apart, about 20 minutes. Drain and chill. Preheat oven to 425. Lightly butter sides and bottoms of 7 muffin cups. Peel and coarsely shred cold cooked potatoes. Gently press about 1/3 cup potato shreds against bottom and side of each muffin cup. Sprinkle with cheeses. Bake about 30 minutes or until lightly browned. Combine horseradish, sour cream, green onions, salt and pepper in a small bowl. Spoon mixture into center of hot potatoes.