Preheat the oven to 350°F, Trim the ends of the zucchinis and slice in half lengthways. Use a spoon to scoop out the center seeds to form a cavity. Place in a single layer on a baking sheet lined with parchment paper and set aside.
Heat the olive oil in a large skillet over a medium-high heat. Add the diced onion and diced bell pepper, and cook, stirring for 4 minutes or until softened. Add the garlic and cook, stirring, for 1 minute or until fragrant.
Add the ground beef to the onion mixture and cook, stirring and breaking up any lumps, until browned all over and no longer pink. Add the marinara sauce and Italian seasoning and continue stirring until heated through. Season to taste with salt and pepper. Remove from the heat.
Spoon the sauce mixture into the hollowed out zucchinis. Transfer to the oven and bake for 20 minutes.
Top the zucchini boats with the shredded mozzarella cheese and parmesan cheese, and bake for a further 10 minutes.
Garnish the zucchini boats with the chopped fresh parsley (if using) before serving.