Heat oil in a large, heavy pot over medium-high heat. Add ½ cup onions, 2 garlic cloves and sauté for 3 minutes. Add tomato and beef broths and chipotle. Bring to a boil. Cover and simmer for 10 minutes.
Meanwhile, combine ground beef, cornmeal, whole milk, egg, salt, pepper, cumin, rice, the remaining onions, garlic cloves and cilantro in a medium bowl.
Mix well. Shape the meat into golf-ball sized balls. Add to the boiling broth and bring to a boil, stirring occasionally.
Reduce heat, cover and simmer until the meatballs are tender, stirring occasionally, about 20 minutes.