Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper or foil.
Slice the zucchinis in half lengthwise and use a spoon to scoop out the centers, creating “boats.” Place the zucchini boats on the prepared baking sheet, and cut side up. Drizzle with olive oil and sprinkle with a little salt.
Bake the zucchini boats in the preheated oven for about 15-20 minutes, until they are just tender.
In a large skillet, heat the remaining olive oil over medium heat.Add the diced onions and sauté until translucent, about 5 minutes.
Add the ground sweet Italian sausage and cook until browned, breaking it up with a spoon as it cooks.
Stir in the minced garlic and cook for another minute until fragrant.Add marinara sauce and Italian seasoning. Season with salt and pepper to taste. Let the sauce simmer for about 10 minutes, allowing the flavors to meld together
Remove the baked zucchini boats from the oven. Pat the insides dry with a paper towel if there is excess moisture. Fill each zucchini boat generously with the sausage bolognese mixture.
Sprinkle the shredded mozzarella cheese evenly over the top of each stuffed zucchini boat.
Return the stuffed zucchini boats to the oven and bake for an additional 10-15 minutes, or until the cheese is melted and bubbly.
Remove from the oven and let cool slightly. Garnish with fresh basil or parsley if desired. Serve warm.