Preheat your oven to 400°F (200°C) and coat a baking dish with cooking spray.
Cut zucchini in half lengthwise and scoop out the flesh to create boats.
Season zucchini shells with Italian seasoning, salt, and pepper.
In a skillet, heat olive oil and cook the sausage until browned. Add onion and garlic, cooking until soft.
Pour in marinara sauce and simmer for 5 minutes.
Fill zucchini shells with sausage mixture and top with mozzarella cheese.
Bake for 25 minutes until zucchini is tender and cheese is bubbly.
Garnish with parsley before serving.