Lightly grease the inside of your slow cooker if needed.
Place the beef tips in the bottom of the slow cooker. Add the sliced mushrooms, chopped onion, and minced garlic on top.
In a bowl, combine the beef broth, Worcestershire sauce, salt, black pepper, dried thyme, and paprika. Stir well, then pour the mixture over the beef and mushrooms.
Cover and cook on LOW for 6–8 hours or on HIGH for 3–4 hours, until the beef is tender and easily pierced with a fork.
In a small bowl, whisk together the cornstarch and cold water until smooth. Stir the mixture into the slow cooker during the last 20–30 minutes of cooking. Cover and allow the sauce to thicken.
For a creamier gravy, stir in the sour cream just before serving.
Taste the gravy and adjust the seasoning if needed. Serve the beef tips warm with your favorite side dishes.
