Cook the pasta shells according to package directions; drain and rinse in cold water. Place mashed potatoes in a large microwave-safe bowl. Cover and microwave on high power for 3-4 minutes, stirring once. Add the minced onion, garlic powder and onion powder. Stir in 1 cup of the cheddar cheese until well blended.
Stuff into the shells. Place in a greased baking dish. Sprinkle with green onions and remaining cheese. Cover and bake at 350 for 20 minutes. Uncover; bake 10 minutes longer or until heated through.