Prepare the patties by mixing together the ground beef, mustard powder, ketchup, Worcestershire sauce, soy sauce, cayenne pepper, egg, and breadcrumbs.
Form the mixture into oval-shaped patties and season with salt and pepper.
Sear the patties in butter in a pan.
In the same pan, sauté the onions and mushrooms until softened.
Add the garlic and thyme and deglaze the pan with white wine.
Make a roux with butter and flour, then add beef stock to create a gravy.
Add the seared patties back to the gravy and let them cook through.
Serve the Salisbury steak with mashed potatoes and garnish with parsley.