Sprinkle roast evenly with pepper; place in a 6-quart pressure cooker. Add mushroom soup and next 3 ingredients.
Cover cooker with lid, and seal securely; place pressure control over vent and tube. Cook over medium-high heat 20 minutes or until pressure control rocks quickly back and forth. Reduce heat to medium-low, and cook 20 more minutes.
Remove from heat; run cold water over cooker to reduce pressure. Carefully remove lid so that steam escapes away from you.
Remove roast, and keep warm. Stir together cornstarch and 2 tablespoons water; add to liquid in pressure cooker. Bring to a boil; cook 1 minute. Serve gravy with roast.