Brown beef and saute onions in butter until golden. Mix meat with onions, catsup, Worcestershire, salt & pepper. Add a little meat stock and cook, covered, over low heat for 15 to 20 minutes. Beat egg yolks until light, whites until stiff. Beat yolks, cream, butter, and garlic powder into mashed potatoes. Gently fold in beaten egg whites. Put meat mixture into casserole and top with potatoes. Sprinkle with Parmesan cheese and bake at 350F. until puffed and browned.