Mix together ground beef, bread crumbs, mushrooms, chopped onions, egg, salt and pepper in a bowl until combined.
Shape the mixture into 4 equal-sized patties.
Fry the patties in a skillet, 5 minutes per side.
Add the broth and sliced onion and mushrooms to the skillet.
Bring the mixture to a bowl then reduce heat to low and cover.
Simmer for about 10-12 minutes or until no longer pink in the center.
Transfer patties to a plate and cover them with foil to keep them warm.
In a separate bowl, combine the cornstarch and the water. Set aside.
Bring the gravy mixture in the skillet to a boil.
Stir the cornstarch mixture into the gravy mixture and cook for about 1-2 minutes or until thickened.
Pour over patties and serve.
Garnish with sprig of rosemary if desired.