Pound rib-eye to ¾ cm thickness using a meat mallet. Slice into finger-width strips across the grain. Pat dry with paper towels, then season generously with salt and pepper. Let sit for 10 minutes at room temperature.

Heat 1 Tbsp oil in a 30 cm skillet over high heat until shimmering. Lay half the beef strips flat in a single layer; cook 30 seconds per side until golden but still pink inside (internal temp 46-48 degrees Celsius). Transfer to a plate.

Add remaining 1 Tbsp oil to the pan and repeat with the second batch of beef. Set aside with the first batch.

Lower heat to medium. Melt butter in the same pan, add sliced onion and cook for 1 minute. Scatter mushrooms in one layer; leave untouched for 2 minutes, then stir and cook 2 more minutes until deep golden. Season lightly with salt and pepper.

Sprinkle flour over the vegetables and stir for 1 minute to cook off the raw taste and create a light roux.

Add 125 ml beef broth, scraping up all the brown bits from the bottom of the pan. When smooth, whisk in remaining broth and Dijon mustard. Simmer for 5 minutes until sauce coats the back of a spoon.

Remove pan from heat completely. Stir in sour cream until the sauce turns velvety and smooth. Do not return to heat or boil.

Slide seared beef and its accumulated juices back into the pan. Set heat to low and warm for 1 minute without further cooking.

Serve stroganoff over buttered egg noodles or tagliatelle. Sprinkle with chopped chives and a final grind of black pepper.