- 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.