01 - In a bowl, toss beef strips with flour, salt, and black pepper until evenly coated.
02 - Heat 1 tablespoon butter and olive oil in a large skillet over medium-high heat. Sear beef strips in batches for 1 to 2 minutes until browned. Remove and set aside.
03 - Reduce heat to medium. Add remaining butter to the skillet. Sauté onions for 3 minutes until translucent, then add garlic and cook for 30 seconds.
04 - Add sliced mushrooms and cook for 5 to 7 minutes until browned and moisture has evaporated.
05 - Stir in tomato paste and Dijon mustard, cooking for 1 minute to blend flavors.
06 - Pour in beef broth and Worcestershire sauce, scraping up browned bits from the skillet. Bring to a simmer and cook for 3 minutes.
07 - Return the beef with any accumulated juices to the pan. Simmer gently for 3 to 4 minutes to meld flavors.
08 - Reduce heat to low. Stir in sour cream until smooth and warmed through. Avoid boiling to prevent curdling.
09 - Plate over egg noodles or rice. Garnish with chopped fresh parsley if desired.