Dice corned beef and cooked potatoes (or substitute coarsely grated raw potato). Chop onion finely. Mix together in a bowl, adding seasonings, add egg and mix more. Put in skillet, coated with oil and form patty 1/2"-3/4" thick, sprinkle with dry mustard. Cook till crispy (about 10 minutes), cover with a large plate and flip over. Sprinkle with mustard and cook till crisp. Serve with a fried egg on top.