Place the beef in a bowl, add the onion and parsley, and mix well. Add the horseradish, ketchup and egg and mix well together; season to taste. Cover, then refrigerate for 1 hour. Using wet hands, divide the mix into 4 evenly sized portions.
Shape each into a ball then, with wet fingers, make a deep indentation into the ball. Place 1 oyster into the hole, then