**Prepare the beef**
Toss the sliced beef with the flour, salt, and black pepper until evenly coated.
**Sear the beef**
Heat 1 tablespoon of olive oil or butter in a large skillet over medium-high heat. Add the beef in batches and sear until browned on both sides, about 1–2 minutes per side. Remove the beef from the skillet and set aside.
**Sauté the vegetables**
In the same skillet, add the remaining olive oil or butter. Sauté the onions and garlic until softened, about 3 minutes. Add the mushrooms and cook for another 5–6 minutes, or until the mushrooms are golden and tender.
**Make the sauce**
Reduce the heat to medium. Pour in the beef broth, Worcestershire sauce, and Dijon mustard (if using). Stir to combine and scrape up any browned bits from the bottom of the pan. Let the sauce simmer for 5 minutes to reduce slightly.
**Add the sour cream and beef**
Lower the heat to medium-low and stir in the sour cream until the sauce is smooth and creamy. Return the seared beef (and any juices) to the skillet. Simmer gently for 5–7 minutes, or until the beef is cooked through and the flavors meld together.
**Serve**
Spoon the beef stroganoff over cooked egg noodles and garnish with fresh parsley. Serve immediately.