In a slow cooker, combine the beef stew meat, chopped onion, minced garlic, and sliced mushrooms.
Pour in the beef broth, Worcestershire sauce, Dijon mustard, paprika, and season with salt and pepper. Stir to combine.
Cover and cook on low for 7-8 hours or on high for 4-5 hours, until the beef is tender.
About 30 minutes before serving, mix the sour cream and flour in a small bowl until smooth. Stir this mixture into the slow cooker and cook on high for an additional 30 minutes to thicken the sauce.
Serve the beef stroganoff over cooked egg noodles or rice, and garnish with fresh parsley.
Notes
This dish can be prepared a day in advance; just reheat before serving.
Add more vegetables like peas or carrots if desired.
For a gluten-free option, substitute flour with cornstarch.