Season the beef: Slice the fillet into thin strips. Season with salt, pepper, and 1 teaspoon of smoked paprika. Toss to coat evenly.
Cook the aromatics: Heat half the oil in a wide frying pan over medium heat. Fry the onion, stirring often, until soft and translucent, 6 to 8 minutes. Add the garlic and mushrooms, increase the heat, and fry for 3 to 4 minutes until the mushrooms are tender. Tip everything onto a plate.
Sear the beef: Add the remaining oil to the pan over high heat. Fry the beef strips until golden brown, 1.5 to 2 minutes total. Work in batches if your pan is small so the strips sear rather than steam.
Build the sauce: Return the onions, garlic, and mushrooms to the pan. Add a splash of brandy and let it boil until almost all reduced. Stir in the sour cream and bring to a gentle simmer.
Finish: Adjust the seasoning with salt, pepper, and a squeeze of lemon juice. Throw in the chopped parsley and remove from the heat. Serve straight away, sprinkled with a pinch of paprika.
