01 - Season the sliced beef evenly 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 beef and set aside on a plate.
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 mushrooms are browned and liquid has evaporated, about 5 minutes.
04 - Stir in minced garlic and cook for 1 minute until fragrant.
05 - Sprinkle flour over the vegetables, stirring constantly 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 sauce begins to thicken, 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 skillet from heat. Stir in sour cream until sauce is smooth and creamy. Taste and adjust seasoning with additional salt and pepper if needed.
09 - Serve immediately over cooked egg noodles. Garnish with chopped fresh parsley.