Whisk broth, Worcestershire, soy sauce, garlic, thyme, pepper, and salt in a bowl.
Add beef, onion, and mushrooms to the slow cooker; toss to distribute evenly.
Pour the sauce over the beef mixture and dot the top with butter.
Cover and cook on Low for 6–7 hours or on High for 3–4 hours, until tender.
Stir cornstarch and water in a cup to make a slurry; pour into the cooker.
Cook covered 15–20 minutes more, until the gravy thickens and turns glossy. Season to taste and serve.