Combine the first 8 ingredients in a bowl, and shape the mixture into 24 (3/4-inch) meatballs.
Heat the oil in a Dutch oven over medium-high heat. Add onion; sauté 4 minutes or until tender. Stir in tomato, water, and broth; bring to a boil. Add celery, carrot, corn, and cumin, if desired. Return to a boil; drop meatballs into pot. Cover, reduce heat, and simmer 30 minutes or until vegetables are tender and meatballs are cooked. Stir in 3 tablespoons cilantro.