Add olive oil to the pan, make sure the pan is hot and sear the steak strips for 3 minutes on each side.
Remove the steak from the pan and allow to rest in a bowl. Add the butter, mushrooms and onion to the pan, season with salt and black pepper and cook for 5 minutes stirring regularly.
Add the flour and stir through, cook for three minutes, stirring constantly.
Add the beef stock and simmer for a couple of minutes. Stir in the crème fraîche and Dijon mustard.
Place the fried beef strips back into the pan and heat for 3 minutes.
Dice the gherkins and add to the pan. Sprinkle a handful of chopped parsley and serve with rice or pasta of choice.