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 and set aside.
03 - Reduce heat to medium. Add the remaining olive oil and butter to the same skillet. Sauté the onions for 2-3 minutes until softened, then add mushrooms. Cook until mushrooms are browned and any liquid evaporates, about 5 minutes.
04 - Stir in the garlic and cook for 1 minute until fragrant.
05 - Sprinkle the flour over the vegetables, stirring well to coat evenly. Cook for 1 minute to remove raw flour taste.
06 - Gradually pour in beef broth, scraping up any browned bits from the bottom of the pan. Add Dijon mustard and Worcestershire sauce. Stir until the sauce begins to thicken, about 3 minutes.
07 - Reduce the 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, then stir in the sour cream until the sauce is smooth and creamy. Adjust seasoning with additional salt and pepper as needed.
09 - Serve hot over cooked egg noodles. Garnish generously with chopped fresh parsley.