Heat the oil and butter in a large pan over medium-low heat. Add the onions and mushrooms, cooking for 7–10 minutes until soft. Set them aside.
Increase the heat to high and add the steak strips to the same pan. Fry for 3–5 minutes until browned.
Pour in the brandy or wine and let it bubble until the alcohol cooks off.
Stir in the beef stock, mustard, smoked paprika, Worcestershire sauce, salt, and pepper. Use a spatula to scrape the bottom of the pan, mixing in all the flavorful bits.
Return the onions and mushrooms to the pan. Cook everything together for 3 minutes until warmed through.
Lower the heat and stir in the crème fraîche or sour cream. Mix gently to prevent the sauce from splitting.
Sprinkle with chopped parsley and remove from heat.
Serve with mashed potatoes or rice and a sprinkle of cornichon on top if using.