In a large mixing bowl, combine ground beef, ground pork, breadcrumbs, Parmesan cheese, minced garlic, parsley, egg, Italian seasoning, salt, and pepper. Mix well until combined.
Preheat your oven to 400°F (if baking meatballs). Alternatively, you can cook them directly in the soup.
Form the mixture into 1-inch meatballs.
Place the meatballs on a baking sheet and bake for 20 minutes or until browned. If cooking directly in the soup, set aside after forming.
In a large pot or Dutch oven, add the chicken or vegetable broth and bring to a boil.
Add diced tomatoes, carrots, celery, onion, dried basil, red pepper flakes (if using), salt, and pepper.
Once the broth is boiling, reduce to a simmer and add the meatballs (either baked or raw).
Let the soup simmer for 25 minutes until the vegetables are tender and the meatballs are cooked through.
Taste and adjust seasonings if necessary.
Serve hot, garnished with fresh basil if desired.