1. Combine cola, next 4 ingredients, and 2 Tbsp. oil in a large zip-top plastic freezer bag. Add roast, turning to coat. Seal and chill at least 8 hours or up to 24 hours. Remove roast from marinade, reserving marinade. Sprinkle roast evenly with pepper.
2. Brown roast 4 minutes on each side in remaining 1 Tbsp. hot oil in a large heavy-duty roasting pan over medium-high heat. Add reserved marinade, stirring to loosen particles from bottom of pan. Bring to a boil. Remove from heat, and cover with heavy-duty aluminum foil.
3. Bake at 300° for 2 1/2 hours. Turn roast, and stir in carrots, potatoes, and onions. Bake 2 more hours or until roast and vegetables are tender. Transfer roast and vegetables to a serving platter. Skim fat from juices in roasting pan.
4. Whisk together cornstarch and 1/2 cup water in a small bowl until smooth. Whisk cornstarch mixture into juices in pan; cook over medium-high heat 5 minutes or until thickened, whisking to loosen particles.
5. Drizzle 1/2 cup gravy over roast. Serve remaining gravy with meat and vegetables.