Prepare the beef: Slice sirloin thinly against the grain. Season with salt and pepper.
Sear the beef: Heat olive oil in a large skillet over medium-high heat. Sear beef in batches for 1–2 minutes until browned. Remove and set aside.
Sauté veggies: In the same skillet, melt butter. Add sliced onion and mushrooms. Cook 6–8 minutes until softened and golden.
Add garlic and flour: Stir in minced garlic and flour. Cook 1 minute to eliminate raw flour taste.
Deglaze and simmer: Pour in beef broth, Worcestershire, and Dijon. Stir and simmer for 5–7 minutes until thickened slightly.
Return beef to pan: Lower heat to low. Stir in cooked beef and any juices.
Add sour cream: Remove skillet from heat and stir in sour cream until smooth. Season to taste.
Serve: Spoon over egg noodles or preferred base. Garnish with parsley and cracked pepper.