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 steak to the same skillet and quickly fry over high heat for 3-5 minutes. When meat is seared on outside with pink and red in the middle take out of skillet and put on cutting board.
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 flavor to the sauce.
Bring mushrooms and onions back to the pan and cook for 3 minutes until everything is heated through and bubbling.
- Slice Steak into thin strips and plate.
Stir in sour cream and dill and take off the heat.
- serve stroganoff sauce over steak or on side.
Serve with veggies, over egg noodles or over potatoes.