Mix together beef, bread crumbs, onion, margarine and salt and pepper. Beat egg until bubbly. Add 2/3 cups milk to meat mixture and mix gently but thoroughly. Shape into 9x12" rectangle on wax paper. Using paper to lift meat, roll into 9" cylinder. Place on a shallow pan and bake at 350'F. for 45 minutes. Prepare instant potatoes. Add butter, onions and salt to potatoes. Beat egg yolks. Add milk to potatoes. Take meat loaf out of oven and frost with mashed potatoes. Return to oven 10 minutes longer. Place cheese on top and put under broiler until cheese melts. Serves 6-8.