Mix ground beef with mustard, Worcestershire Sauce, Salt and Pepper.
Form in into oval shaped patties.
Place in oven in oven proof frying pan at 325 degrees.
In the meantime, saute the onions just until they are softened, about 10 minutes.
Add them to the hamburger steaks in the oven and continue to bake for another 10-20 minutes, making sure your hamburger steaks are cooked to at least medium rare.
See above for optional sauces.
Serve with mashed potatoes.