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; sear roast, 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.