In a large bowl, combine onion, parsley, egg, garlic, salt, pepper, and bread crumbs. Stir well. Stir in grated Parmesan, ground turkey, and Italian sausages. For each meatball, scoop 1 heaping teaspoon of the turkey mixture and roll into a ball. Place on a baking sheet.
Bring chicken broth to a boil in a large saucepan set over medium-high heat. Add the meatballs to the broth and cook at a gentle boil until the meatball are cooked through, 8 to 10 minutes. Stir in fresh spinach leaves.
In a medium bowl, whisk together 2 eggs and 2 tablespoons grated Parmesan. Stir the soup in a circular motion and slowly drizzle in the egg mixture.
Gently stir the soup with a fork until the egg forms thin strands, about 1 minute. Season to taste with salt and freshly ground black pepper. Serve immediately. If desired, top each serving with grated Parmesan cheese.