Preheat the oven to 400°F and generously coat a large rectangular baking dish with cooking spray. Slice each zucchini lengthwise and trim away the stem ends, then use a spoon to gently hollow out the center flesh of each zucchini half. Season the hollowed zucchini shells with dried Italian seasoning, salt, and pepper, and arrange them snugly in the prepared baking dish. Warm the olive oil in a large skillet set over medium-high heat; add the sausage and sauté for 4–5 minutes, breaking it apart as it browns. Stir in the diced onion and continue cooking for about 4 minutes until the onion softens, then add the minced garlic and cook for an additional 30 seconds. Adjust seasoning with salt and pepper as needed. Pour in the marinara sauce, stir well, and let the mixture simmer gently for 5 minutes. Evenly distribute the sausage mixture into the prepared zucchini shells and sprinkle each generously with shredded mozzarella. Transfer the dish to the oven and bake for 25 minutes, or until the zucchini is tender and the cheese is fully melted and lightly browned. Garnish with chopped parsley before serving.