Put the ground beef in a deep bowl, add the egg, mustard, Worcestershire sauce and 5 tablespoons of beef broth. Mix thoroughly. Now add breadcrumbs, as well as salt and spices. Should form a meatloaf texture that can be formed into patties, may need more breadcrumbs to get to this texture if it is too wet and doesn't stay together.
In your large skillet add 1 tbsp of butter, fry the onion and mushrooms for 5-7 minutes to brown over low medium heat. Stir so as not to burn.
Add flour and fry for 3-4 minutes, stirring occasionally, over medium heat to create a roux.
Pour in the beef broth, stirring, bring to a boil. Break all the lumps of flour. Shape the minced meat into steaks and fry them in another pan in the remaining butter, about 5 minutes per side, to brown the outsides over medium heat.
Pour sauce with mushrooms and broth into the meat pan, (or transfer meat patties into mushroom gravy pan. Cover with a lid and cook on low heat for another 20 minutes.
If your sauce gets too thick for your liking, add a splash of broth to thin and serve over mashed potatoes.