Mix the ground beef, half the chopped onion, breadcrumbs, egg, Worcestershire sauce, salt, and pepper in a bowl. Form the mixture into four oval patties.

Melt 1 tablespoon of butter in a large skillet over medium heat. Add the patties and cook for 4 to 5 minutes per side until browned on both sides. Remove the patties to a plate.

Add the remaining tablespoon of butter to the same skillet. Stir in the mushrooms and the rest of the chopped onion. Cook for 5 minutes until the mushrooms soften.

Sprinkle the flour over the mushrooms and stir for 1 minute. Slowly pour in the beef broth while stirring to avoid lumps. Return the patties to the skillet and simmer for 10 minutes until the gravy thickens.

While the gravy simmers, cook the egg noodles in a pot of boiling water according to package directions. Drain the noodles.

Serve the Salisbury steaks and mushroom gravy over the warm noodles. Sprinkle with chopped parsley.