01 - Season beef strips generously with salt and freshly ground black pepper on both sides.
02 - Heat 1 tbsp vegetable oil in a large skillet over medium-high heat. Sear half the beef strips until browned, approximately 1–2 minutes per side. Remove beef from skillet and set aside. Repeat with remaining beef, adding more oil if needed. Reserve all seared beef with accumulated juices.
03 - Reduce heat to medium. Add butter to the same skillet. Sauté chopped onions for 2 minutes until translucent. Add sliced mushrooms and cook for 5 minutes until softened and golden brown. Stir in minced garlic and cook for 1 minute until fragrant.
04 - Sprinkle flour evenly over mushroom mixture, stirring constantly to coat. Cook for 1 minute to remove raw flour taste. Add white wine if using and allow to reduce for 1 minute, stirring to incorporate.
05 - Gradually pour in beef broth while stirring continuously to prevent lumps from forming. Bring mixture to a gentle simmer, stirring until sauce begins to thicken slightly.
06 - Stir in Dijon mustard followed by sour cream until fully combined and smooth. Reduce heat to low and simmer gently for 3–4 minutes. Do not allow sauce to boil or sour cream may separate.
07 - Return beef strips and any reserved juices to the skillet. Simmer for 2–3 minutes, just until beef is heated through and sauce reaches desired consistency. Taste and adjust seasoning with additional salt and pepper if necessary.
08 - Serve immediately over cooked egg noodles or steamed rice. Garnish with chopped fresh parsley.