Place potatoes in a large saucepan and cover with water. Bring to a boil. Reduce heat; cover and simmer for 15-20 minutes or until tender. Drain.
In a large bowl, mash potatoes with sour cream, cottage cheese and milk; stir in onions. Transfer to an 8-in. square baking dish coated with cooking spray; sprinkle with cheddar cheese. Bake at 375° for 5-10 minutes or until heated through and cheese is melted. Yield: 5 servings.