01 - Season beef strips generously with salt and freshly ground black pepper on all sides.
02 - Heat 1 tbsp vegetable oil in a large skillet over medium-high heat. Sear half the beef strips until browned, about 1-2 minutes per side. Remove beef from pan and set aside. Repeat with remaining beef, adding the remaining tablespoon of oil if needed. Set all seared beef aside.
03 - Reduce heat to medium. Add butter to the skillet, then 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 the 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.
05 - Gradually pour in beef broth while stirring continuously to prevent lumps from forming. Bring mixture to a gentle simmer, stirring occasionally.
06 - Stir in Dijon mustard and sour cream until fully combined and smooth. Simmer gently for 3-4 minutes; do not allow sauce to boil or cream may separate.
07 - Return beef and any accumulated juices to the pan. Simmer for 2-3 minutes until beef is heated through and sauce has thickened to desired consistency. Taste and adjust seasoning with additional salt and pepper if needed.
08 - Serve immediately over cooked egg noodles or steamed rice, garnished generously with chopped fresh parsley.