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 for 4 minutes, stirring once. Add the minced onion, onion powder and garlic powder. Stir in 1 cup of cheese until well blended.
Using a tablespoon, stuff potato mixture in shells. Place in a greased 13-in by 9-in 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.