Method
Season the beef with salt and pepper. Sauté in butter and a teaspoon of oil at a high heat until brown but not so long as to let too much juice come out of the meat. Put to one side.
In a bigger pan, brown the onion in butter and a teaspoon of oil, and then add the mushrooms. Add the reserved steak. Let in cook for 5 minutes.
Add the soy sauce and heated brandy (if using) and singe.
In a bowl, mix the ketchup, mustard, salt and pepper and flour dissolved in milk. Add the mixture to the steak and mushrooms and cook for 5–8 minutes on a medium heat until the sauce incorporates and thickens.
Serve with fondant potatoes.