Pat the steaks proper dry with kitchen roll. Rub with olive oil and season both sides loads with salt and pepper.
Get a large pan on medium-high heat. Chuck in the butter. When it's sizzling, lay the steaks in and cook 3-4 minutes each side for medium-rare. Don't move them about!
Take steaks out, stick them on a plate, and cover loosely with foil.
Turn heat down to medium. Bung in the garlic and cook 30 seconds till it smells gorgeous.
Pour in the stock and scrape up all those brown bits stuck to the pan.
Add the cream and let it bubble for 2-3 minutes till it thickens up a bit.
Chuck in the Parmesan, mustard, and chilli flakes if using. Stir till the cheese melts.
Squeeze in the lemon juice and give it a stir.
Put the steaks back in with any juices from the plate. Spoon sauce over them and let them warm through for a minute or two.
Scatter parsley on top and serve with loads of that sauce!