Walt Disney's Roast Beef Hash

1    medium onion, finely chopped 
2    tablespoons butter 
1    cup finely chopped cooked prime rib roast 
1    medium raw potato, peeled and finely chopped 
1    cup beef stock 
1    fried egg 

1.  Preheat oven to 350F degrees. 
2.  Saute onion in melted butter until tender but not browned; combine sauteed onion with beef, stock and raw potato and put into a buttered 4-cup baking dish. 
3.  Bake in preheated oven for 1 hour. 
4.  Remove from oven and shape mixture into an oval patty; place in a hot buttered skillet. 
5.  Saute until lightly browned on both sides; meanwhile, in a second skillet, fry the egg. 
6.  Top hash with egg and serve immediately.