01 - Season the sliced beef thoroughly with salt and pepper on all 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 from skillet 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 has evaporated, approximately 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 of the pan. Add Dijon mustard and Worcestershire sauce. Stir continuously until the sauce begins to thicken, about 3 minutes.
07 - Reduce heat to low. Return the beef and any accumulated juices to the pan. Simmer gently for 2-3 minutes until the beef is just cooked through.
08 - Remove from heat and stir in the sour cream until the sauce is smooth and creamy. Adjust seasoning with additional salt and pepper if needed.
09 - Serve immediately over cooked egg noodles, garnished with chopped fresh parsley.