Combine roast, celery, salt, pepper, mustard, and water in a large Dutch oven. Bring to a boil; cover and simmer 2 hours.
Sauté potatoes in oil in a heavy skillet over medium heat until browned. Add potatoes to roast, reserving pan drippings in skillet. Sauté onion in reserved pan drippings until lightly browned. Add onion and tomatoes to roast. Cover and cook over medium heat 30 minutes or until vegetables are tender. Transfer roast to a platter, and serve with vegetables.