Prepare the Meatballs: Soak the chopped bread in milk for 5 minutes until soft. Squeeze out excess milk and mash it into a paste.

In a large bowl, combine the ground beef, eggs, onion, cilantro, cumin, salt, pepper, and cayenne pepper. Mix gently until well combined.

Form the mixture into meatballs about 1.5 inches in diameter and set aside.

Heat olive oil in a large pot or Dutch oven over medium-high heat. Brown the meatballs in batches, cooking for about 10 minutes per batch. Remove from the pot and set aside.

Make the Soup Base: In a small pot, add tomatoes, onion, and garlic. Cover with water and bring to a boil. Simmer for 15 minutes until soft. Drain and blend with chipotle chilies until smooth.

Cook the Vegetables: Pour the blended tomato-chipotle sauce into the large pot. Add chicken stock, potatoes, carrots, zucchini, and green beans. Bring to a simmer and cook for 20-30 minutes, until the potatoes and carrots soften.

Finish the Soup: Return the meatballs to the pot and let simmer for another 10 minutes until fully cooked.

Serve hot, garnished with fresh cilantro and red pepper flakes. Serve with white rice if desired.