For the Meatballs:

1. Preheat the oven to 400°F. Line a baking sheet with parchment paper or foil sprayed with non-stick cooking spray.
2. In a large bowl, combine the torn bread and grated onion. Let sit for one minute to soften.
3. Add parmesan cheese, whisked egg, garlic, Italian seasoning, salt, and pepper. Stir until well combined.
4. Add the ground pork and gently mix with your hands until just combined. Avoid overmixing to keep the meatballs tender.
5. Use a teaspoon to portion out small amounts of the mixture, then roll into balls. Place them on the baking sheet, close together but not touching.
6. Bake for 25-30 minutes, or until golden brown and cooked through to an internal temperature of 160°F. Set aside. (This step can be done a few days ahead.)

For the Soup:

1. Heat olive oil in a large soup pot or Dutch oven over medium heat. Add carrots, celery, and shallot/onion. Season with salt and pepper.
2. Sauté for 8-10 minutes, stirring occasionally, until the vegetables are tender. If needed, add a splash of chicken stock and cover briefly to steam the vegetables.
3. Pour in the chicken stock and add the parmesan cheese rind (if using). Increase the heat to bring the liquid to a boil.
4. Add the orzo, reduce the heat to medium-low, and simmer for about 20 minutes, or until the pasta is al dente.
5. Stir in the cooked meatballs and bring the soup back to a gentle boil.
6. Add the chopped spinach and cook for 2-3 minutes, or until wilted.
7. In a small bowl, whisk together the egg and grated parmesan cheese. Stir the soup vigorously to create a whirlpool effect, then slowly stream in the egg mixture, stirring constantly to create delicate threads of egg.
8. Remove the parmesan rind (if used). Let the soup rest for 10-15 minutes to thicken slightly before serving.