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 over low heat to prevent the sauce from splitting. Sprinkle parsley and take off the heat. Serve with mounds of mashed potatoes or rice.
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Notes
If the sour cream taste is too strong for you, replace a half of it with heavy cream.
Do not use low fat sour cream as it tends to produce a thin sauce and sometimes cause the sauce to split.