01 - Season the sliced beef evenly 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 from skillet and set aside with any accumulated juices.
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 any liquid has evaporated, approximately 5 minutes.
04 - Stir in the minced garlic and cook for 1 minute until fragrant, being careful not to burn it.
05 - Sprinkle flour evenly over the vegetables, stirring constantly to coat everything. Cook for 1 minute to remove the raw flour taste.
06 - Gradually pour in beef broth while scraping up any 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 reserved juices to the pan. Simmer gently for 2-3 minutes until the beef is just cooked through. Do not overcook.
08 - Remove skillet from heat completely. Stir in sour cream until the sauce is smooth and creamy. Taste and adjust seasoning with additional salt and pepper if needed.
09 - Serve immediately over hot cooked egg noodles. Garnish generously with chopped fresh parsley.