In a large mixing bowl, combine ground beef, Italian sausage, breadcrumbs, grated Parmesan cheese, fresh parsley, egg, minced garlic, salt, and black pepper. Mix gently with your hands until just combined, being careful not to overmix.

Roll the mixture into approximately 1-inch meatballs. You should get about 30-35 meatballs. Place them on a baking sheet.

Heat 1 tbsp olive oil in a large Dutch oven or soup pot over medium-high heat. Carefully add meatballs in batches, searing them on all sides until browned. They don't need to be cooked through. Remove browned meatballs to a plate and set aside.

In the same pot (add a little more olive oil if needed), reduce heat to medium. Add diced onion, carrots, and celery. Sauté for 5-7 minutes, or until vegetables begin to soften.

Add minced garlic, dried oregano, and red pepper flakes (if using) and cook for another minute until fragrant.

Pour in the crushed tomatoes and chicken broth. Bring the mixture to a simmer, scraping up any browned bits from the bottom of the pot.

Carefully add the seared meatballs back into the pot. Reduce heat to low, cover, and simmer for 15 minutes to allow the meatballs to cook through and flavors to meld.

Stir in the orzo pasta. Continue to simmer, uncovered, for another 8-10 minutes, or until the orzo is al dente. Stir occasionally to prevent the orzo from sticking.

Just before serving, stir in the fresh spinach until it wilts, about 1-2 minutes. Taste and adjust seasoning with salt and pepper as needed.

Ladle the hot soup into bowls. Garnish with fresh chopped basil and a generous sprinkle of grated Parmesan cheese.