Preheat the oven to 400 degrees F. Coat a large rectangular baking with with cooking spray.
Cut the zucchini in half lengthwise, then trim off the stem ends. Use a spoon to carefully scoop the flesh out of the zucchinis.
Sprinkle the Italian seasoning, salt and pepper over the zucchini shells. Arrange the zucchini in the baking dish.
Heat the olive oil in a large pan over medium high heat. Add the sausage and cook for 4-5 minutes, breaking up the meat with a spatula.
Add the onion and cook for an additional 4 minutes or until onion is softened. Add the garlic and cook for 30 seconds.
Season the sausage and vegetable mixture with salt and pepper.
Pour the marinara sauce into the pan and bring to a simmer; cook for 5 minutes.
Spoon the meat mixture evenly into the zucchini shells, then top with the shredded cheese.
Bake for 25 minutes, or until zucchini is tender and cheese is melted and golden brown.
Sprinkle with parsley, then serve.