Preheat oven to 425°F. Halve zucchini lengthwise and scoop out the centers, leaving ¼-inch-thick walls. Finely chop and reserve ½ cup of the scooped zucchini flesh.

Arrange zucchini cut-side up on a baking sheet. Brush with olive oil and season with ¼ tsp salt and the black pepper. Roast for 8–10 minutes. Carefully blot any liquid inside the boats.

Meanwhile, brown sausage in a large skillet over medium-high heat for 5–7 minutes, breaking it into crumbles. Drain excess grease if needed. Add onion and reserved zucchini flesh; cook for 4–5 minutes, until softened and excess moisture evaporates.

Stir in garlic and Italian seasoning; cook for 30–60 seconds. Add 1 cup marinara and simmer uncovered for 3–5 minutes, until thickened. Taste and add the remaining salt as needed.

Reduce oven temperature to 400°F. Spread the remaining ½ cup marinara in a 9×13-inch baking dish. Arrange zucchini boats over the sauce and divide the filling evenly among them. Top with mozzarella and Parmesan.

Bake uncovered for 12–18 minutes, until zucchini is fork-tender and cheese melts. Broil for 1–2 minutes for additional browning, if desired, watching closely. Garnish with parsley and serve with optional pasta and extra marinara.