Heat oil in large stockpot over medium heat. Add onion: cook until tender, stirring occasionally, 3 to 5 minutes. Add cumin; cook and stir 1 minute. Pour in beef broth and water. Increase heat to high.
Pinch ground beef into teaspoon-sized meatballs and place on a plate. When soup is boiling, slide in meatballs. Stir well, and simmer until meat is cooked through, 3 to 4 minutes. Stir in beans, tomatoes and corn: cook until thoroughly heated. Stir in lime juice and serve with chopped fresh cilantro, reduced-fat cheddar cheese, nonfat plain yogurt, or seeded and chopped jalapenos, if desired.