01 -
Pat the beef strips dry and season with salt and pepper. Heat the oil in a large frying pan over high heat. Sear the beef in two batches for 1–2 minutes per batch, turning once, until browned outside. Transfer to a plate; it will finish cooking in the sauce.
02 -
Lower the heat to medium. Melt the butter in the same pan and cook the onion for 5–6 minutes until softened. Add mushrooms and cook for 6–8 minutes until their released liquid has evaporated and they begin to colour.
03 -
Sprinkle flour and paprika over the vegetables. Stir for 1 minute, then gradually pour in the broth while stirring to prevent lumps. Scrape up browned bits from the pan.
04 -
Bring to a gentle simmer and cook for 3–4 minutes until the sauce lightly coats a spoon. Stir in the mustard.
05 -
Return the beef and its juices to the pan. Simmer gently for 2–3 minutes, until the beef is cooked to your preferred doneness, without prolonged boiling that can make the strips tough.
06 -
Remove from the heat. Stir a few spoonfuls of warm sauce into the sour cream, then stir this mixture back into the pan. Warm gently only if needed; do not boil after adding sour cream. Serve immediately. Rice or noodles may be served alongside and are not included in these quantities.
07 -
Cool leftovers promptly and refrigerate within two hours for up to three days. Reheat gently, stirring, until hot throughout; add a splash of water if the sauce thickens.