Pat the beef dry and season it well. Heat 1 Tbsp of the oil in a large frying pan over a high heat until it’s just smoking.
Sear the beef in two batches, about a minute a side, until browned outside and still pink within. Lift onto a plate and set aside. Don’t crowd the pan.
Add the remaining oil and the mushrooms to the same pan over a high heat. Leave them undisturbed for 2 minutes to colour, then stir and cook a further 3 minutes until browned. Tip out onto the plate with the beef.
Turn the heat to medium and add the butter and onion. Cook for 5 minutes until soft, then add the garlic for a minute.
Stir in the flour and paprika and cook for a minute. Add the stock gradually, stirring, then the mustard and Worcestershire sauce. Simmer for 3–4 minutes until thickened.
**Take the pan off the heat.** Put the sour cream in a small bowl, stir in two spoonfuls of the hot sauce to loosen it, then stir that mixture back into the pan.
Return the beef, the mushrooms and any resting juices to the pan and warm through over a low heat for 30 seconds. Do not let it boil.
Season, scatter with parsley, and serve immediately over egg noodles, rice or mash.