In a large mixing bowl, combine the beef with the seasoned breadcrumbs, dry mustard, crushed beef bouillon cube, Worcestershire sauce, ketchup and then season with salt and pepper.
Form the beef mixture into about 6 oval patties.
Heat a large skillet over medium-high heat. Add the butter and the olive oil to the skillet.
Fry the patties until they are no longer pink in the middle.
After that, remove the patties and set aside. Pour off any extra grease from the patties.
Reduce the heat to medium. Add the mushrooms and onions to the same pan and cook until the onions are tender and golden brown.
Then, add the beef stock, Worcestershire, and ketchup. Stir and cook to reduce. Make a slurry with the cornstarch and a little water or broth and add it to the gravy to help thicken it.
Return the patties to the skillet and coat them with the onion and mushroom gravy.