Heat the oil and butter in a large frying pan. Press the peppercorns on both sides of the steaks to form an even coating.
Place the mushrooms and steaks in the hot pan and cook the steaks for 2 minutes each side for rare or 3–4 minutes each side for medium to well done, stirring the mushrooms occasionally. Remove the steaks from the pan and keep warm.
Add the marsala to the pan and cook for 1 minute, then stir in the soured cream and cook over a low heat until the sauce thickens. Season with salt, pour the sauce over the steaks and serve with mangetout.