In a large bowl, combine the beef, breadcrumbs, egg, Worcestershire sauce, salt and pepper, using your clean hands. Divide into 6 equal pieces and form each one into oval patties.
In a large skillet add the olive oil and butter and heat until melted over medium-high heat. Add the meat patties and fry on both sides until no longer pink in the middle, about 5 minutes per side. Transfer the patties to a plate.
Reduce the heat to medium and add the sliced onions and cook until softened. Add the mushrooms, if using, and cook for a few more minutes until the onions are golden brown.
Add the beef broth, tomato paste, soy sauce, and Worcestershire sauce and stir. In a small bowl whisk the cornstarch with a bit of water or beef broth and add to the skillet. Whisk until the sauce thickens, for about 2 minutes.
Add the meat back to the pan and spoon gravy over the top. Cook for another minute until the meat patties warm through.
Garnish with parsley and serve.