In slow cooker, stir together cornstarch and 2 tablespoons cold water until smooth. Add carrots and onions; season with salt and pepper, and toss.
Sprinkle roast with 1 teaspoon salt and 1/2 teaspoon pepper; place on top of vegetables, and drizzle with Worcestershire. Cover; cook on high, 6 hours (or on low, 10 hours).
Transfer roast to a cutting board; thinly slice against the grain. Place vegetables in a serving dish; pour pan juices through a fine-mesh sieve, if desired. Serve roast with vegetables and pan juices.
Oven method: Preheat oven to 350 degrees. In step 1, use a 5-quart Dutch oven or ovenproof pot with a tight-fitting lid; add 2 cups water. Cover; bake until roast is tender, 3 1/2 to 4 hours. Proceed with step 3.