Add the beef, breadcrumbs, egg, diced onion, garlic, Worcestershire sauce, mustard, paprika, oregano, coriander and a pinch of salt and pepper into a bowl. Mix well with your hands.
Divide the meat mix into 8 even pieces and form them into mini steaks. Heat a skillet over MED-HIGH heat and drizzle with olive oil. Sear the steaks on each side for 2 minutes until they are browned.
In the same pan, melt the butter. Add the sliced onions and mushrooms. Sauté for 5 minutes.
Sprinkle the flour into the pan. Mix well. Pour in the beef stock. Reduce to a simmer and stir until the gravy thickens.
Place the steaks back in the pan with the gravy and cook for a further 5 minutes.