Prep beef: Pound rib-eye; slice into strips; season with salt and pepper.
Sear batch 1: Heat 1 Tbsp oil high; sear half the beef 30 s per side; transfer to plate.
Sear batch 2: Add remaining oil; repeat with rest of beef; set aside.
Brown veg: Lower heat; melt butter; add onion 1 min, then mushrooms; cook 4 min until golden.
Make roux: Sprinkle flour; stir 1 min.
Deglaze: Add ½ cup broth; scrape fond; when smooth add rest of broth and Dijon; simmer 5 min.
Finish sauce: Remove from heat; stir in sour cream until silky.
Warm beef: Return beef and juices; heat 1 min on low.
Serve: Ladle over buttered noodles; top with chives.