Prepare the beef. Pat the beef strips dry with paper towels and season lightly with salt and pepper.
Sear the beef. Heat olive oil in a large skillet over medium-high heat. Add the beef in a single layer, searing for 1–2 minutes per side until browned. Do not overcrowd the pan. Remove the beef to a plate and set aside.
Cook the aromatics. In the same pan, reduce heat to medium. Melt butter, then add onions and cook until soft and golden, about 5 minutes. Stir in garlic and cook 30 seconds more until fragrant.
Add mushrooms. Toss in the sliced mushrooms and sauté until they release their juices and start to brown, about 5–6 minutes.
Create the sauce base. Sprinkle flour over the mushroom mixture and stir well to coat. Slowly pour in beef broth, stirring constantly to avoid lumps.
Season the sauce. Add Worcestershire sauce and Dijon mustard. Simmer gently for 5 minutes until the sauce thickens slightly.
Finish with creaminess. Lower heat to medium-low. Stir in sour cream until smooth and creamy. Taste and adjust seasoning with salt and pepper.
Return the beef. Add the seared beef (and any juices from the plate) back into the pan. Stir gently and simmer 2–3 minutes until the beef is just cooked through.
Serve. Spoon the beef stroganoff over warm egg noodles. Garnish with fresh parsley and enjoy immediately.