Preheat oven to 400°F. Spray a broiler pan or a 13 x 9 inch baking dish with cooking spray.
Use a teaspoon to scoop out the zucchini seeds and 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.
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.
In a small bowl, mix together ricotta, egg, 1 Tablespoon Parmesan and 1/2 teaspoon dried Italian seasoning.
To Assemble: Sprinkle 1/2 of shredded mozzarella cheese onto bottom of each zucchini boat. Dollop ricotta mixture over cheese dividing it evenly between the zucchini halves then top with Italian sausage sauce. Sprinkle the tops with reserved 1 cup mozzarella and 2 Tablespoons grated Parmesan cheese.
Place into the oven and bake for 20-25 minutes. (Test for doneness with the tip of a knife.)
Serve immediately with reserved warm marinara sauce on the side, if desired.
Notes
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 pork
1 1/2 teaspoon salt
1 teaspoon dried Italian seasoning
1 teaspoon fennel seed
1 teaspoon granulated garlic
1 teaspoon onion powder
1/2-1 teaspoon crushed red pepper flakes (optional adjust to taste or omit)