Prepare the beef: Slice the beef thinly across the grain. Pat dry, season with salt and pepper, then toss lightly in flour.
Sear the beef: Heat 1 tbsp oil + 1 tbsp butter in a skillet. Sear beef quickly in batches until browned but not fully cooked. Remove and set aside.
Cook vegetables: In the same skillet, add remaining oil and butter. Sauté onion until soft, then add mushrooms and cook until golden. Stir in garlic.
Make the sauce: Pour in beef broth, Dijon mustard, and Worcestershire sauce. Stir well, scraping browned bits from the pan. Simmer for 5 minutes.
Finish with sour cream: Lower heat, stir in sour cream until smooth. Return beef to skillet, simmer gently for 2–3 minutes until warmed through.
Serve: Spoon Stroganoff over cooked egg noodles or pasta. Garnish with parsley. Serve immediately.