Preheat your oven to 400ºF.
Wash, peel and dice the potatoes. Add them to a pot of salted water and boil for 15 minutes, or until tender.
While the potatoes boil, prepare the filling by starting to brown the ground beef and diced onion.
Once the meat is browned, add the frozen vegetable mix, the ketchup, Worcestershire sauce, flour, garlic powder and water. Combine well, until a sauce forms. Cook for 2-3 minutes.
When your potatoes are done, drain and return them to the pot. Add the butter, milk and mash until smooth. Stir in the egg yolk.
Scoop the potatoes onto the meat mixture and spread evenly.
Bake for 30 minutes, until the potatoes begin to brown. Remove from the oven and allow to rest for 10 minutes before serving.
Serve with a nice green salad.
