Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper.

Cut each zucchini in half lengthwise. Using a spoon, scoop out the seeds and some flesh to create "boats," leaving about 1/4-inch thick walls. Don't discard the flesh – chop it up for the filling.

Season the hollow zucchini with salt and pepper, then place them cut-side up on the prepared baking sheet.

In a large skillet, heat olive oil over medium heat. Add diced onion and cook until translucent, about 3-4 minutes.

Add minced garlic and cook for another 30 seconds until fragrant.

Add Italian sausage to the skillet, breaking it up with a wooden spoon. Cook until browned, about 5-7 minutes.

Stir in the chopped zucchini flesh and marinara sauce. Cook for another 2-3 minutes.

Remove from heat and stir in half the mozzarella cheese and the breadcrumbs.

Fill each zucchini boat generously with the sausage mixture.

Top with remaining mozzarella and Parmesan cheese.

Bake for 20-25 minutes, or until the zucchini is tender and the cheese is golden brown.