Beef

Heat olive oil in a non-stick frying pan, add sliced onion and cook over medium until completely softened, adding a splash of water if sticking
Crush in garlic and cook 2–3 minutes
Add butter; once it foams add mushrooms and cook about 5 minutes until softened
Season and transfer cooked onions and mushrooms to a plate
Put plain flour in a bowl, season with salt and pepper, then toss steak in the seasoned flour
Add floured steak to the pan and fry 3-4 minutes until well coloured, adding oil if required
Return cooked onions and mushrooms to the pan
Whisk crème fraîche, mustard and beef stock together and pour into the pan
Cook over medium heat for about 5 minutes
Scatter with parsley and serve