Heat 1 tbsp olive oil in a non-stick frying pan then add 1 sliced onion and cook on a medium heat until completely softened, around 15 mins, adding a little splash of water if it starts to stick.
Crush in 1 garlic clove and cook for 2-3 mins more, then add 1 tbsp butter.
Once the butter is foaming a little, add 250g sliced mushrooms and cook for around 5 mins until completely softened.
Season everything well, then tip onto a plate.
Tip 1 tbsp plain flour into a bowl with a big pinch of salt and pepper, then toss 500g sliced fillet steak in the seasoned flour.
Add the steak pieces to the pan, splashing in a little oil if the pan looks dry, and fry for 3-4 mins, until well coloured.
Tip the onions and mushrooms back into the pan. Whisk 150g crème fraîche, 1 tsp English mustard and 100ml beef stock together, then stir into the pan.
Cook over a medium heat for around 5 mins.
Scatter with some chopped parsley, and serve with pappardelle or rice.
