Pat the beef dry with paper towels. Toss with 1 tbsp flour, 1/2 tsp salt, and 1/4 tsp black pepper until evenly coated.
Heat 1 tbsp oil in a large heavy skillet over high heat until shimmering. Sear the beef in two batches, 60 to 90 seconds per side, until deep golden-brown. Remove to a plate.
Reduce heat to medium. Add 2 tbsp butter. Cook diced onion for 4 to 5 minutes until soft and translucent.
Add sliced mushrooms. Cook 6 to 7 minutes until golden-brown and the pan is completely dry with no visible liquid.
Add garlic and stir for 30 seconds until fragrant.
Sprinkle remaining 1 tbsp flour over the pan. Stir to coat everything and cook for 1 minute.
Pour in beef broth, Worcestershire sauce, and Dijon mustard. Scrape all the browned bits from the pan bottom. Simmer 4 to 5 minutes until the sauce coats the back of a spoon.
Remove pan from heat. Stir in sour cream until the sauce is smooth and even in color.
Return beef and resting juices to the pan. Warm on low heat for 2 to 3 minutes. Do not boil. Beef must reach 71°C (160°F) internally.
Serve over cooked egg noodles, garnished with parsley.