Pound the rib-eye to 1/3-inch thickness. Slice into strips about 3 inches long and season both sides with kosher salt and pepper. Start a pot of salted water for pasta now.

Heat 1 tablespoon of oil in a large skillet over high heat until shimmering. Sear half the beef strips for 30 seconds per side until browned but not cooked through. Transfer to a plate. Add remaining oil and repeat with second batch.

Reduce heat to medium. Add butter to the same skillet. Once melted, add the onion and cook for 1 minute. Add mushrooms and cook for 4 minutes, stirring occasionally, until deep golden brown and moisture has cooked off.

Sprinkle flour over the mushroom mixture and stir well. Cook for 1 minute. Pour in 1/2 cup beef broth, scraping up browned bits. Once thickened, add remaining 1 1/2 cups broth and Dijon mustard. Simmer for 5 minutes until sauce coats the back of a spoon.

Remove skillet completely from heat. Stir in room-temperature sour cream until smooth and silky. Do not return to high heat after this step.

Return seared beef and accumulated juices to the skillet. Set over low heat and stir gently for about 1 minute until beef is just warmed through. Taste and adjust salt.

Drain pasta and toss with a small pat of butter. Ladle stroganoff over noodles and garnish with fresh chives.