Heat oil and butter in a heavy skillet and cook onions and mushrooms over medium-low heat for 7-10 minutes. Remove and set aside.
Add cut steak to the same skillet and quickly fry over high heat for 3-5 minutes.
Add brandy and continue cooking until alcohol burns off, add stock, mustard, bay leaf, salt and pepper. Scrape the bottom of the pan with a spatula to release all the brown bits, they will add much flavour to the sauce.
Bring mushrooms and onions back to the pan and cook for 3 minutes until everything is heated through and bubbling.
Stir in sour cream and parsley and take off the heat.
Serve with mounds of mashed potatoes or rice.