In a large bowl, combine ground beef, rice, mint, cilantro, onion, egg, garlic, cumin, oregano, salt, and pepper.
Mix gently (do not overwork) and shape into 1-inch meatballs. Set aside.
Heat olive oil in a large pot over medium heat. Sauté onion and garlic until fragrant (2-3 mins).
Add carrots and celery, cooking for 5 minutes until slightly softened.
Pour in broth, water, diced tomatoes, cumin, oregano, and bay leaf. Bring to a boil.
Gently drop meatballs into the boiling broth. Reduce heat to a simmer.
Cover and cook for 25-30 minutes, until meatballs are cooked through and rice is tender.
Add zucchini (if using) in the last 10 minutes of cooking.
Season with salt and pepper to taste.
Discard the bay leaf. Ladle soup into bowls.
Garnish with fresh cilantro and serve with lime wedges.