Prepare the Beef. Season the beef with salt and pepper, then brown it in a skillet over medium-high heat until it's golden brown on all sides. Transfer the beef to the slow cooker.
Cook the Vegetables. In the same skillet, sauté the onions and garlic until they're softened and fragrant. Add the mushrooms and continue to cook until they're golden brown. Transfer the vegetables to the slow cooker.
Add the Remaining Ingredients. To the slow cooker, add the beef broth, Worcestershire sauce, paprika, and thyme. Stir to combine.
Cook. Cover and cook on low for 6-8 hours or on high for 3-4 hours, until the beef is tender and cooked through.
Add the Sour Cream. Stir in the sour cream until it's well combined and creamy.
Serve. Serve the Beef Stroganoff over cooked egg noodles, garnished with fresh parsley if desired.