Add onion, carrots, garlic, celery, and salt and pepper to the Dutch oven and saute until softened, about 5 minutes. Add the tomato paste and wine to the vegetables and cook, stirring, about 1 minute. Add the stock, diced tomatoes, thyme, and bay leaf. Return the browned short ribs and any juices that have accumulated back into the Dutch oven. Add orange zest. Cover with a heavy lid and place in the oven and braise for 3 hours or until the meat is very tender.