Divide the meat mixture into 4 pieces and shape each into an oval patty about 3/4 inch thick

Cook the patties in a large skillet over medium heat, turning occasionally, until browned on the outside

Drain any fat from the pan and keep the patties over medium heat

Add the onion, beef broth, and mushrooms

Heat to a boil, then reduce the heat to medium

Cover the pan and simmer until the beef is done, about 10 minutes

Remove the patties from the pan and cover to keep warm

Heat the broth mixture to a boil again

In a small bowl, whisk the cornstarch and water together, then pour into the broth

Boil and stir for about 1-2 minutes, until it thickens slightly

Serve immediately with the sauce over the beef