In a large mixing bowl, combine Italian style breadcrumbs and milk, mashing them until smooth. Incorporate ground beef, Italian sausage, Parmesan cheese, egg, salt, and pepper until just blended. Form mixture into 1-inch meatballs, yielding about 40-45, and transfer to a plate.

Heat olive oil in a large pot over medium-high heat until shimmering. Add meatballs in batches for around 5 minutes, turning until golden-brown. Use a slotted spoon to transfer cooked meatballs back to the plate.

In the same pot, add chopped onion, celery, and carrots, sautéing for about 7-8 minutes until softened. Add minced garlic and stir for an additional minute until fragrant.

Pour in chicken broth, add Italian seasoning and reserved meatballs. Bring to a boil, then add acini de pepe pasta. Reduce heat, simmer uncovered for 10 minutes, until pasta is al dente.

Fold in fresh baby spinach, stirring until it wilts. Adjust seasoning with additional salt and pepper as desired. Serve hot, garnished with grated Parmesan cheese.