Heat the olive oil in a non-stick frying pan then add the sliced onion and cook on a medium heat until completely softened, so around 15 mins, adding a little splash of water if they start to stick at all. Crush in the garlic and cook for a 2-3 mins further, then add the butter. Once the butter is foaming a little, add the mushrooms and cook for around 5 mins until completely softened. Season everything well, then tip onto a plate. 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.