Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
In a large mixing bowl, combine the ground beef, ground pork, Parmesan cheese, breadcrumbs, parsley, minced garlic, egg, salt, and black pepper. Mix until just combined and shape into small meatballs (about one inch in diameter).
Place the meatballs on the prepared baking sheet and bake in the preheated oven for about 15 minutes or until lightly browned.
While the meatballs are baking, heat the chicken broth in a large pot or Dutch oven over medium-high heat. Add the chopped onion, diced carrots, and diced celery. Cook for about 5 minutes, until the vegetables start to soften.
Stir in the dried oregano and red pepper flakes (if using) and continue to simmer for an additional 5 minutes.
Once the meatballs are done, carefully add them to the pot with the broth and vegetables. Simmer for about 10 minutes to allow the flavors to meld.
Stir in the chopped kale or spinach and cook for an additional 5 minutes, until the greens are wilted and tender.
Taste and adjust the seasoning with salt and pepper as needed.
Serve the soup hot, garnished with additional grated Parmesan cheese if desired.