In a large skillet, brown the beef pieces over medium-high heat for about 5 minutes. Transfer to the crockpot.
In the same skillet, sauté the chopped onion and minced garlic until softened, about 3 minutes. Add them to the crockpot with the browned beef.
Add the sliced mushrooms, beef broth, Worcestershire sauce, Dijon mustard, salt, and pepper to the crockpot. Stir to combine all ingredients.
Cover and cook on low for 6 hours or high for 3 hours until the beef is tender.
About 30 minutes before serving, combine the sour cream and flour in a small bowl until smooth. Stir this mixture into the stroganoff in the crockpot. Let it cook for the remaining 30 minutes, uncovered, to thicken.
Meanwhile, cook the egg noodles or rice according to package instructions. Drain and set aside.
Serve the beef stroganoff over the cooked noodles or rice, and garnish with fresh parsley if desired.