Break the bread into small pieces and pulse in a food processor until finely crumbled, then mix in the water and let it rest briefly
In a bowl, gently combine the ground beef, ground pork, Parmesan, egg, Italian seasoning, salt, pepper, and soaked bread until just mixed
Form the mixture into approximately 1-inch meatballs and lay them on a plate
Heat oil in a large Dutch oven over medium-high heat and sear the meatballs on all sides until browned, about 2 minutes per side, then remove and set aside
Using the same pot, toss in the chopped carrots, celery, and onion and cook in the meatball remnants for about 5 minutes until soft
Add minced garlic, Italian seasoning, salt, bay leaf, and red pepper flakes if you like, cooking for another minute until fragrant
Pour in the chicken broth and add the Parmesan rind, scraping the bottom of the pot to lift any browned bits, then bring everything to a simmer
Let the soup cook for 5 minutes until the vegetables are nearly tender
Stir in the small pasta and browned meatballs, cover, reduce heat to medium, and simmer for 10 minutes or until the pasta is just tender
Finally, fold in the fresh spinach until wilted, then serve the soup hot
