Mash potatoes with butter and salt. Heat oil in skillet, add onion, beef and pepper. Saute for 5 minutes. Butter 1 quart casserole. Put half of the potatoes in the bottom of the dish. Add the meat and cover with the balance of the potatoes. Smooth the surface. Cover with the beaten egg and sprinkle with cheese. Bake in 350 degree oven until the surface is golden about 30 minutes.