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. Add half the beef strips and sear until browned, about 1–2 minutes per side. Remove browned beef and set aside. Repeat with remaining beef, adding more oil if needed. Set all seared beef aside with accumulated juices.
03 - Reduce heat to medium. Add butter to the skillet and let it melt. Add chopped onions and sauté 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 vegetables. Cook for 1 minute to remove raw flour taste. If using white wine, pour it in now and let it reduce for 1 minute, stirring to deglaze the pan.
05 - Gradually pour in beef broth while stirring continuously to prevent lumps from forming. Bring the mixture to a gentle simmer and let it cook for 2–3 minutes until slightly thickened.
06 - Stir in Dijon mustard and sour cream until fully incorporated and smooth. Reduce heat to low and simmer gently for 3–4 minutes, stirring occasionally. Do not let the sauce boil to prevent curdling.
07 - Return seared beef and any accumulated 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 needed.
08 - Serve the stroganoff hot over cooked egg noodles or steamed rice. Garnish with chopped fresh parsley and serve immediately.