01 - Season the sliced beef thoroughly with salt and pepper on both sides.
02 - Heat 1 tbsp olive oil in a large skillet over medium-high heat. Sear the beef in batches for 1-2 minutes per side until browned but not cooked through. Remove beef and set aside.
03 - Reduce heat to medium. Add remaining olive oil and butter to the same skillet. Sauté onions for 2-3 minutes until softened, then add mushrooms. Cook until browned and liquid evaporates, about 5 minutes.
04 - Stir in the garlic and cook for 1 minute until fragrant.
05 - Sprinkle flour over the vegetables, stirring well to coat evenly. Cook for 1 minute to remove raw flour taste.
06 - Gradually pour in beef broth while scraping up browned bits from the bottom. Add Dijon mustard and Worcestershire sauce. Stir until sauce thickens, about 3 minutes.
07 - Reduce heat to low. Return beef and any accumulated juices to the pan. Simmer gently for 2-3 minutes until beef is just cooked through.
08 - Remove from heat. Stir in sour cream until sauce is smooth and creamy. Adjust seasoning with salt and pepper as needed.
09 - Serve immediately over cooked egg noodles. Garnish with chopped fresh parsley.