01 - Season the thinly sliced beef with salt and black pepper.
02 - Heat 1 tablespoon of unsalted butter and the olive oil in a large skillet set over medium-high heat. Sear the beef in batches for 1 to 2 minutes per side until just browned. Remove the beef from the skillet and set it aside on a plate.
03 - Reduce the skillet heat to medium. Add the remaining 1 tablespoon of unsalted butter. Sauté the finely chopped onion for 2 to 3 minutes until it becomes translucent. Add the minced garlic and cook for an additional 30 seconds until fragrant.
04 - Add the sliced mushrooms to the skillet. Cook for 5 to 6 minutes, stirring occasionally, until the mushrooms soften and turn golden brown.
05 - Sprinkle the all-purpose flour over the cooked vegetables and mushrooms. Stir well for 1 minute to toast the flour and eliminate its raw taste.
06 - Gradually pour in the beef broth while stirring constantly to prevent any lumps from forming. Incorporate the Worcestershire sauce and Dijon mustard.
07 - Allow the sauce to simmer for 3 to 4 minutes, stirring occasionally, until it has slightly thickened.
08 - Reduce the heat to low. Gently stir in the sour cream until the sauce is smooth and fully incorporated; avoid bringing the sauce to a boil. Return the seared beef and any accumulated juices to the skillet. Stir gently to coat the beef with the sauce and allow it to warm through for 2 to 3 minutes.
09 - Taste and adjust seasoning as needed. Serve the Beef and Mushroom Stroganoff immediately over cooked egg noodles, garnished with freshly chopped parsley.