Season the beef fillets with salt and pepper and set aside.
Heat the butter in a large frying pan. Add the onions and sauté for 2 minutes then add the mushrooms, cook until both are soft. Add the mustard and stir it into the vegetables, coating thoroughly. Pour over the stock and leave to simmer until the liquid has reduced by about half. Stir in the crème fraiche and set aside for a few minutes.
In another large frying pan heat the vegetable oil. When it is smoking, add the beef cubes. Stir constantly – until the meat is browned on all sides. You only want it to cook for 1 minute, at the most.
If you want to flambé the beef, put the brandy in a ladle and heat it over the gas. When it starts to burn off (you will see the fumes), tip it very gently towards the flame and it will ignite (CAUTION: Keep your face and hair away from the flames.). Immediately pour this over the beef and give it a quick stir.
Return the onion and mushroom sauce to the heat and add the beef to the pan. Season to taste, sprinkle over the parsley and serve.