Make the Meatballs:

1. Combine Ingredients: In a large bowl, mix the ground beef, egg, breadcrumbs, Parmesan, parsley, garlic, salt, and pepper until just combined. Avoid overmixing to keep the meatballs tender.
2. Form Meatballs: Roll the mixture into small, bite-sized meatballs (about 1 inch in diameter). Place them on a plate or baking sheet.
3. Brown the Meatballs: Heat 1 tablespoon of olive oil in a large pot or Dutch oven over medium heat. Add the meatballs in batches and cook until browned on all sides, about 5 minutes. Remove the meatballs and set them aside.

Prepare the Soup Base:

4. Sauté Vegetables: In the same pot, add the diced onion, carrots, and celery. Sauté for 5-7 minutes, or until softened. Add the garlic and cook for another minute.
5. Add Broth and Tomatoes: Pour in the broth, diced tomatoes, and tomato sauce. Stir in the oregano, basil, and bay leaf. Bring the soup to a simmer.

Cook the Soup:

6. Add Meatballs: Gently return the browned meatballs to the pot. Simmer for 20-25 minutes, allowing the flavors to meld and the meatballs to cook through.
7. Cook the Pasta or Rice: Add the pasta or rice during the last 10 minutes of cooking. Stir occasionally to prevent sticking.
8. Add Greens: Stir in the spinach or kale during the last 5 minutes of cooking. Taste and adjust seasoning with salt and pepper.

Serve:

9. Garnish: Ladle the soup into bowls and top with grated Parmesan and fresh parsley or basil. Serve with crusty bread or garlic toast for dipping.