Polish Perogi Casserole

Cook noodles as directed on package to al dente. Saute onions in 1 stick of butter (may use more if desired). Butter 13x9 inch pan with remaining 1/2 stick butter. Mix sauteed onions with mashed potatoes. Layer noodles, then potatoes, then cheese, and repeat, ending with cheese. Cover with foil and bake in a 350° oven for 25-30 minutes or until cheese is melted. (top with sour cream and garnish with chives/green onion)