Cut the steak into thin strips and season lightly with salt and pepper.
Sear the steak in a little oil over medium-high heat until browned, working in batches if needed. Remove from the pan.
Lower the heat, add the butter, then cook the onion and garlic until softened and lightly browned.
Stir in the flour and cook for about 1 minute.
Pour in the beef broth and stir until combined.
Stir in the sour cream and Dijon mustard. Bring to a simmer, reduce to medium-low, and cook for 3–5 minutes until thick and creamy.
Return the steak to the sauce and simmer for 1–2 minutes.
Taste and season with salt and pepper as needed. Serve over cooked egg noodles.