Preheat your oven to 350°F. Line a baking sheet with aluminum foil and position an oven-proof roasting rack on top. Generously coat the rack with nonstick cooking spray to prevent sticking.

In a large mixing bowl, whisk together the egg, chives, sage, and garlic until well blended. Add the ground beef, Italian sausage, breadcrumbs, Parmigiano Reggiano, and salt. Use your hands to thoroughly mix the ingredients until they are evenly distributed. Shape the mixture into tablespoon-sized meatballs, about 1 inch in diameter, forming approximately 50 meatballs in total. Arrange the meatballs on the prepared rack and bake for 15 to 18 minutes, or until they are lightly browned and cooked through. Set the meatballs aside.

In a large soup pot or Dutch oven, heat the olive oil over medium heat. Add the diced onions, carrots, and celery, stirring frequently until the vegetables are softened, which should take around 8 minutes. Pour in the chicken broth, beef broth, water, and white wine (if using). Drop in the bay leaf and season with salt and pepper. Bring the mixture to a boil. Once boiling, add the pasta and cook at a gentle boil, uncovered, until the pasta reaches an al dente texture, about 8 to 10 minutes (or as directed on the package). Taste the soup and adjust the seasoning if necessary. Lower the heat, add the spinach and meatballs to the pot, and let it simmer for a few minutes until the spinach wilts and the meatballs are heated through. Ladle the soup into bowls and garnish with freshly grated Parmigiano Reggiano.

The soup can be frozen for up to 3 months, but hold off on adding the pasta until you’re ready to reheat it. To reheat, defrost the soup in the refrigerator for about 12 hours, then warm it on the stovetop over medium heat until it simmers. Add the pasta and cook until it reaches the desired tenderness.