In a 5- to 6-quart slow cooker, stir together cornstarch and 2 tablespoons broth until smooth.
Add remaining broth, tomato paste, potatoes, carrots, onion, and Worcestershire. Season with salt and pepper and toss.
Season roast with 1 teaspoon salt and ½ teaspoon pepper, and rub with garlic.
Place on top of vegetables. Cover and cook on high until roast is fork-tender, 5 hours (or 8 hours on low).
Transfer roast to a cutting board; thinly slice against the grain.
Place vegetables in a serving dish; skim fat from pan juices, then pour through a fine-mesh sieve, if desired.
Serve roast and vegetables drizzled with juices.
