Slice steak (600 g) into 6mm strips, discard excess fat, season with salt and pepper.
Heat 1 tbsp oil (2 tbsp) in a large skillet on high. Sear half the beef (600 g) 30 sec each side until browned, remove to a plate (still slightly raw inside). Repeat with remaining oil (2 tbsp) and beef (600 g).
Reduce to medium high. Melt butter (40 g), cook onions (1) until soft, add mushrooms (300 g) and cook until golden. Stir in flour (2 tbsp), cook 1 min.
Add half the broth (500 ml) stirring and scraping the pan, then the rest; reduce to thicken slightly. Stir in sour cream (200 ml), Dijon (1 tbsp) and paprika.
Simmer then reduce to medium until sauce reaches pouring-cream consistency; adjust salt and pepper. Return beef (600 g) with juices, simmer 30 sec–1 min, remove immediately. Serve over rice or pasta with parsley.