Prepare the beef

• Cut the beef into thin strips and season with salt and pepper.
• Heat the oil in a frying pan over high heat and sear the beef in batches until browned.
• Set the beef aside on a plate.

Cook the onions and mushrooms

• Melt the butter in the same pan and soften the onion.
• Add the mushrooms and cook until golden and their liquid has evaporated.

Build the sauce

• Sprinkle the flour over the vegetables and stir for a minute.
• Pour in the beef stock and stir until slightly thickened.
• Stir in the Dijon mustard and sour cream.

Finish and serve

• Return the beef and its juices to the pan and warm through gently.
• Adjust the seasoning and scatter with chopped parsley.
• Serve hot, with rice, pasta or mashed potatoes.