Pulse the tomato juice, oregano, green onion, chipotle peppers, garlic, brown sugar and cumin in a blender until smooth. Pour over the beef roast and marinate in the refrigerator for at least 4 hours or overnight. Remove the beef from the marinade and pat dry using paper towels. Reserve the marinade remaining in the bowl.
Preheat oven to 325˚F. Heat the oil in a large, ovenproof Dutch oven set over medium-high heat. Brown the beef all over. Pour the reserved marinade over the beef. Cover and transfer to the oven. Roast, turning once, for 1 hour. Add the sweet potato and parsnips or plantains. Roast, covered, for 2 hours or until beef and vegetables are fork tender. Add the red pepper and roast, uncovered for an additional 15 minutes.
Transfer beef to a cutting board and tent with foil. Rest for 10 minutes before slicing. Use a slotted spoon to transfer the vegetables to a serving bowl. Ladle the pan juices into a gravy boat or heat proof pitcher. Serve meat and vegetables with the pan juices on the side.
For a thicker gravy, strain the juices into a pot set over medium heat. Stir together 2 tbsp each cornstarch and water until dissolved. Whisk into simmering juices and bring to a boil; cook, stirring, until thickened.