Put the butter and a little olive oil in a frying pan. When hot, add the steak, seasoned first with salt and pepper on both sides. Cook for about 4-6 minutes on each side, depending on thickness and how well-done you like the steak. As it cooks, spoon some the melted butter over the steak.
When ready remove the steak from the pan and let it rest. Add the onion, crushed garlic and mushrooms to the pan. Cook for a couple of minutes then add the whisky. Continue to cook for a couple more minutes then add the mustard, stock and creme fraiche. Season to taste.
Heat through then serve the sauce over the steaks.
Nutritional information is approximate and a guideline only.