Combine Meatball Mixture
In a large mixing bowl, mix the ground beef, 1 large egg, uncooked rice, breadcrumb (if using), 1 teaspoon salt, ½ teaspoon pepper, and oregano. Gently form the meat into golf ball–sized meatballs. Set aside.
Sauté Aromatics
Heat 2 tablespoons olive oil in a dutch oven or large stockpot over medium-high heat.
Add chopped onion (and garlic, if desired). Sauté for about 3–4 minutes, stirring occasionally, until onions turn translucent.
Build the Broth
Pour in the diced tomatoes. Stir and let simmer for 2 minutes.
Slowly add the 4 cups of chicken stock while stirring to distribute heat evenly.
Add Meatballs
Bring the mixture to a low boil, then carefully place the meatballs into the pot, one by one. Reduce the heat to medium so it remains a gentle simmer rather than a rolling boil.
Incorporate Vegetables
Add the sliced carrots, chopped zucchini, and green beans (if using). Season with extra salt and pepper to taste.
Cover the pot partially and let the soup cook for about 20–25 minutes, or until the rice inside the meatballs is fully cooked and carrots are tender.
Finish and Serve
Taste the soup, adjusting seasoning if needed.
Ladle the soup into bowls and garnish with fresh cilantro. Serve hot with warm tortillas or crusty bread.