Preheat the oven to 350 degrees F. Spray a 13x9-inch baking pan with nonstick spray.
Use a small spoon to scrape out the insides of each zucchini half - the pulp. Don't scrape the zucchini all the way to the skin- just make a deep pocket where you'll be able to put the stuffing. Save the pulp and chop it.
Heat a large skillet to medium heat and spray with nonstick spray. Add the zucchini pulp, onion, garlic and sausage. Cook together 4 to 6 minutes, stirring often, until the onions are softened and the sausage is cooked through. Pour the mixture through a fine sieve to drain the excess moisture. Transfer the meat mixture to a medium bowl.
In a small bowl, mix the cheeses together. Remove two tablespoons of the cheese and set aside. Add the rest of the cheese to the meat mixture, along with the marinara sauce and basil. Add salt and pepper, as desired.
Divide the meat mixture between the hollowed out zucchini, filling and mounding into each one. Set the stuffed zucchini into the prepared pan and sprinkle the reserved cheese on top.
Bake 30 minutes, or until baked through and tender.
These are delicious on their own, but they're also good served over herbed rice.
If you are preparing this recipe as gluten-free, just be sure to use sausage and marinara sauce that are known to be GF.