Pat beef dry, season with most salt and pepper, and sear in hot oil on all broad sides until deeply browned.
Transfer beef to the cooker. Soften onion briefly, add garlic and tomato paste, then deglaze with 1/2 cup broth and scrape everything into the crock.
Add remaining broth, Worcestershire, thyme, bay leaf, potatoes and carrots around the thawed roast. Cover.
Cook on LOW for 8 to 10 hours until a fork twists easily in the thickest part; avoid repeatedly lifting the lid.
Move beef and vegetables to a platter. Discard bay leaf, stir slurry into the juices and cook on HIGH 15 to 25 minutes until thickened.
Rest beef 10 minutes, slice or pull across the grain, and spoon gravy over beef and vegetables.