Preheat the oven to 400°F and lightly grease a large rectangular baking dish with cooking spray. Halve the zucchinis lengthwise and remove the stem ends, then gently scoop out the inner flesh with a spoon to form hollow shells. Sprinkle the zucchini with Italian seasoning, salt, and pepper, and arrange them in the prepared dish. In a large skillet set over medium-high heat, warm the olive oil and sauté the sausage, breaking it apart as it cooks for about 4-5 minutes. Stir in the diced onion and continue to cook until softened, approximately 4 minutes more. Add the minced garlic and sauté for 30 seconds, then season the mixture with additional salt and pepper. Incorporate the marinara sauce, bring to a gentle simmer, and cook for 5 minutes. Distribute the sausage mixture evenly among the zucchini shells and sprinkle with mozzarella cheese. Bake in the oven for 25 minutes or until the zucchinis are fork-tender and the cheese is melted and golden. Finish with a sprinkle of chopped parsley before serving.