Thinly slice the beef sirloin steak against the grain. Slice the mushrooms and onion. Mince the garlic.

Heat 1 tablespoon of butter in a large skillet over medium-high heat. Add the sliced beef and cook for 2-3 minutes, or until browned on all sides. Remove the beef from the skillet and set aside.

In the same skillet, add the remaining tablespoon of butter. Add the onions and cook for 2-3 minutes, or until they begin to soften. Add the garlic and mushrooms, and cook for another 3-4 minutes, or until the mushrooms are tender.

Sprinkle the flour over the cooked vegetables and stir to combine. Cook for 1-2 minutes to form a roux. Gradually add the beef broth, stirring constantly to avoid lumps. Bring the mixture to a simmer.

Return the cooked beef to the skillet. Stir in the Worcestershire sauce and Dijon mustard. Allow everything to simmer for 5-7 minutes until the sauce thickens and the beef is tender.

Lower the heat to medium-low and stir in the sour cream. Continue to cook for an additional 2-3 minutes, until the sauce is creamy and smooth. Season with salt and black pepper to taste.

Serve the Beef Stroganoff over cooked egg noodles, rice, or mashed potatoes. Garnish with fresh parsley if desired.