01 - Pat the beef strips dry with paper towels. Season with salt and pepper, then dust with the flour to coat evenly.
02 - Heat 1 tbsp butter and olive oil in a large skillet over medium-high heat. Sear the beef in two batches until browned but still pink inside, about 1–2 minutes per side. Remove beef to a plate and set aside.
03 - Reduce heat to medium. Add the remaining 1 tbsp butter to the same skillet. Sauté onions for 3–4 minutes until translucent. Add mushrooms and cook for 5–6 minutes, stirring occasionally, until golden and softened.
04 - Stir in minced garlic and cook for 1 minute until fragrant.
05 - Add tomato paste, stir to coat vegetables, then pour in beef broth, Worcestershire sauce, and mustard. Scrape up any browned bits from the pan.
06 - Bring to a simmer and let the sauce reduce slightly, about 5 minutes.
07 - Lower the heat to low. Whisk in sour cream until smooth and heat gently (do not boil). Adjust seasoning with salt and pepper.
08 - Return beef and any juices to the pan. Stir to coat and heat through for 2–3 minutes.
09 - Serve hot over cooked egg noodles. Garnish with chopped parsley if desired.