In a large bowl, combine the 3/4 lb ground chicken, 1/2 lb sweet Italian chicken sausage, 2/3 cup panko breadcrumbs, 1/2 cup Parmesan cheese, 3 tablespoons fresh dill, 2 cloves garlic, 1 large egg, 3 tablespoons whole milk, 1 teaspoon kosher salt, and 1/2 teaspoon black pepper. Mix by hand until just combined and roll into 1-inch meatballs.
Heat 2 tablespoons olive oil in a large skillet over medium-high heat. Brown the meatballs in batches, turning to color two or three sides. They do not need to cook through.
In the same skillet, add 2 tablespoons olive oil, 1 yellow onion, 3 carrots, and 3 celery stalks. Cook 6 to 8 minutes until softened, then add 4 cloves garlic and cook 1 minute more. Pour in 1/2 cup dry white wine and scrape up the browned bits.
Transfer the vegetables and wine to the slow cooker. Add 10 cups chicken stock and 2 bay leaves, then nestle in the meatballs.
Cover and cook on LOW for 6 to 7 hours or HIGH for 3 to 4 hours.
Stir in 1 cup orzo, switch to HIGH, cover, and cook 15 to 20 minutes until tender.
Remove the bay leaves. Stir in 5 oz baby spinach and let wilt about 2 minutes. Add 3 tablespoons fresh dill and the juice of 1/2 lemon. Season with kosher salt and black pepper to taste.
Serve in wide bowls topped with extra Parmesan.