Prepare beef: Season beef with salt, pepper, and paprika. Optional: brown beef in olive oil in a skillet for extra flavor. Transfer to crockpot.
Add vegetables: Add sliced mushrooms, chopped onion, and minced garlic to the crockpot.
Add liquids and seasonings: Pour in beef broth, Worcestershire sauce, and sprinkle in dried thyme. Stir gently to combine.
Slow cook: Cover and cook on low for 6–7 hours or high for 3–4 hours, until beef is tender.
Thicken sauce: About 30 minutes before serving, mix sour cream and flour (if using) together, then stir into the crockpot. Cook until sauce is creamy and thickened.
Serve: Spoon over cooked egg noodles, rice, or mashed potatoes. Garnish with fresh parsley.