Prepare all ingredients: slice the beef, chop the onion, and slice the mushrooms.
In a large skillet over medium-high heat, melt butter and sauté the beef in batches until browned (about 4–5 minutes). Remove and set aside.
In the same skillet, add more butter if needed, then sauté onions and mushrooms until golden (about 5 minutes). Add garlic and cook for another minute.
Sprinkle flour over vegetables and stir well. Gradually add beef broth while stirring until smooth; bring to a simmer.
Stir in sour cream and Dijon mustard until combined, then return beef to skillet and let simmer for an additional 5 minutes.
Serve generously over egg noodles or rice, garnished with parsley.
