Tip the flour into a bowl with a big pinch of salt and pepper, then toss the steak in the seasoned flour. Add the steak pieces to the pan, splashing in a little oil if the pan looks particularly dry, and fry for 3-4 mins, until well coloured. Tip the onions and mushrooms back into the pan. Whisk the crème fraîche, mustard and beef stock together, then pour into the pan. Cook over a medium heat for around 5 mins. Scatter with parsley, then serve with pappardelle or rice.