Brown beef in 3 Tbs. butter, add onions and sauté until golden. Mix meat and onions with ketchup and Worcestershire sauce. Season to taste with salt & pepper. Add as little beef stock and cook covered over low heat for 15 to 20 minutes. Beat egg yolks until light, whites until stiff. Beat yolks, cream remaining, 1/2 cup 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 350 degrees until puffed and browned.