Preheat and prep pans: Heat your oven to 400°F (200°C). Lightly grease a 9x13-inch baking dish with 1 tablespoon olive oil.

Prep the zucchini: Trim the ends, slice each zucchini in half lengthwise, and use a spoon to scoop out the centers, leaving about 1/4 inch border. Reserve about half of the scooped flesh and chop it finely.

Season the boats: Brush the zucchini shells with the remaining 1 tablespoon olive oil. Sprinkle with salt and pepper. Arrange them cut side up in the baking dish.

Brown the sausage: Heat a large skillet over medium heat. Add the Italian sausage, breaking it up with a spatula. Cook until browned and cooked through, about 6–8 minutes. Drain excess fat if needed, leaving about 1 tablespoon in the pan.

Sauté aromatics: Add the diced onion to the skillet and cook 2–3 minutes until softened. Stir in the garlic and cook 30 seconds until fragrant.

Add zucchini and sauce: Stir in the chopped zucchini flesh, Italian seasoning, and red pepper flakes (if using). Cook 2–3 minutes to soften and reduce moisture. Pour in the marinara, season with salt and pepper, and simmer 2–3 minutes. Remove from heat. Stir in 1/4 cup Parmesan.

Fill the boats: Spoon the sausage mixture evenly into the zucchini shells, mounding slightly.

Top with cheese: Sprinkle the mozzarella and remaining 1/4 cup Parmesan over the boats.

Bake: Cover loosely with foil and bake for 15 minutes. Remove foil and bake another 8–10 minutes, until the zucchini is tender and the cheese is golden and bubbly.

Finish and serve: Let rest 5 minutes. Garnish with chopped basil or parsley. Taste and add a pinch of salt or pepper if needed. Serve warm.