In a frying pan heat oil and butter over medium heat, saute onions and mushrooms with a pinch of salt and pepper until mushrooms are cooked and onions are tender for approximately 5 minutes.

Remove the mushrooms and onions from the pan and set aside.

To the same pan add steak, salt and pepper and quickly fry for approximately 3 minutes until sealed.

Add brandy and scrape the bottom of the pan with a spatula to release the flavour into the sauce, add the beef stock, mustard, bay leaf, bring the onions and mushrooms back to the pan, bring everything to a boil and cook for 5 minutes over low heat.

Add sour cream, stir to combine and simmer for a couple of minutes over gentle heat and take it off the heat.

Garnish with chopped parsley or dill and serve over mashed potatoes.