Get Ready: Preheat your oven to 400°F (200°C) and lightly spray a 9x13 inch casserole dish with cooking spray.
Mix Meatballs: In a large mixing bowl, combine all the meatball ingredients: ground meat, Italian sausage, shredded mozzarella, Parmesan, shredded zucchini, egg, dried minced onion, dried minced garlic, dried basil, and salt. Mix everything thoroughly with your hands or a rubber spatula until just combined.
Form & Bake First Round: Scoop out about 24 meatballs (a large cookie scoop makes this super easy!) and arrange them evenly in your prepared casserole dish. Bake for 30 minutes, or until the meatballs are almost completely cooked through.
Drain & Top: Carefully remove the casserole dish from the oven and gently drain any excess cooking liquid. Pour the sugar-free marinara sauce evenly over the meatballs, then sprinkle generously with the remaining shredded cheese blend.
Finish Baking: Return the casserole to the oven and bake for an additional 10-15 minutes, or until the cheese is beautifully melted and bubbly. For an extra golden-brown top, you can broil for 2-3 minutes at the very end, but keep a close eye on it to prevent burning!
Serve & Enjoy: Let the casserole cool slightly before serving. This wholesome dish is delicious on its own or paired with a crisp side salad!