Preheat oven to 375°F. Lightly mist a 13" x 9" baking sheet with cooking spray.
Prepare the zucchini boats by cutting the zucchini in half length-wise and then use a small spoon to gently scoop out the interior. Leave a thin layer of zucchini in the bottom and sides of the boats to keep the boat sturdy and to prevent the skin from breaking. Place the boats in the baking dish.
Finely dice the zucchini filling and the Roma tomato and set aside.
Heat the olive oil over medium heat. Add the garlic, onion, and bell pepper and cook for 2 to 3 minutes, or until the vegetables begin to soften.
Add the chicken sausage and cook until no longer pink, about 3 to 5 minutes.
Add the diced zucchini-tomato filling. Season with Italian seasoning and salt and pepper. Cook 2 to 3 minutes.
Drain any excess liquid from the pan. Then add the marinara and stir to incorporate. Cook 1 to 2 minutes and remove from heat. I prefer to drain my pan a second time after adding the sauce.
Scoop the cooked filling into the zucchini boats. I prefer a healthy amount of stuffing, so I tend to overfill mine a bit. You will have leftover filling. Top the boats with mozzarella cheese, if desired.
Bake the zucchini boats for 15 to 20 minutes, or until the cheese layer is lightly golden brown and the zucchini is lightly softened. Be careful not to over-bake, as soggy zucchini is not appetizing!