Season beef with salt and pepper.
Heat olive oil in a large skillet over high heat. Sear beef quickly in batches until browned. Remove and set aside.
Add butter to the same skillet. Cook mushrooms until golden and moisture evaporates.
Add onion and sauté until soft. Stir in garlic and cook 30 seconds.
Sprinkle flour (if using) and stir well.
Add beef broth, Dijon mustard, and Worcestershire sauce. Simmer until slightly reduced.
Lower heat and stir in sour cream until smooth.
Return beef to the skillet and warm gently (do not boil).
Garnish with parsley and serve hot.