Preheat oven to 400°F. Spray a broiler pan or a 13x9-inch baking dish with cooking spray.
Prep the Zucchini: Use a teaspoon to scoop out the seeds and part of the zucchini flesh leaving at least a 1/3 inch border. Arrange halves side by side on broiler pan. Season to taste with salt and black pepper. Set aside.
Make the Sausage Filling: In a large skillet cook sausage and onion over medium high heat for 5-8 minutes or until no pink remains. (You can add olive oil but the sausage will likely have ample fat for cooking.) Add garlic to mixture cooking for 1 minute longer. Drain any excess fat from the pan.
To the skillet add 2 cups marinara sauce, tomato paste, 1 teaspoon Italian seasoning and crushed red pepper flakes, if using. (Omit if using spicy sausage or personal preference.) Simmer for 2-3 minutes or just until the tomato paste has melted and the sauce is fully combined. Remove from heat.
Ricotta: In a small bowl, stir together the ricotta, egg, 1 Tablespoon Parmesan and 1/2 teaspoon dried Italian seasoning.
To Assemble: Divide half of the shredded mozzarella cheese between the zucchini sprinkling onto bottom of each zucchini boat. Next, use a spoon to dollop the ricotta mixture over the cheese dividing it evenly between the zucchini halves
Top with Italian sausage mixture. Sprinkle the tops with reserved 1 cup mozzarella and 2 Tablespoons grated Parmesan cheese.
Transfer the dish to the oven and and bake for 22-25 minutes. (Test for doneness with the tip of a knife, it should easily glide through.)
Serve immediately with reserved warm marinara sauce on the side, if desired.
Homemade Italian Sausage: To make your own sausage using ground pork, turkey or chicken for this recipe: Mix the following together and proceed with the recipe browning together with diced onion and garlic in Step #3.1 1/2 pounds of ground pork, turkey or chicken1 1/2 teaspoon salt1 teaspoon dried Italian seasoning1 teaspoon fennel seed1 teaspoon granulated garlic or garlic powder1 teaspoon onion powder1/2-1 teaspoon crushed red pepper flakes (Optional adjust to taste or omit).