Preheat the oven to 375°F (190°C).
Wash the zucchinis and cut each in half lengthwise.
Using a spoon, scoop out the flesh from each zucchini half, leaving about a 1/4-inch thick shell. Chop the scooped-out flesh and set aside.
Place the zucchini shells on a baking sheet lined with parchment paper.
Heat the olive oil in a large skillet over medium heat.
Add the chopped onion and minced garlic to the skillet and sauté for 2-3 minutes until translucent.
Add the ground beef to the skillet and cook until browned, breaking it apart with a spoon, about 5-7 minutes.
Stir in the chopped zucchini flesh and diced tomato, cooking for another 3-4 minutes until the mixture is heated through.
Season the mixture with dried oregano, dried basil, salt, and black pepper. Stir well to combine.
Remove the skillet from heat and stir in half of the shredded mozzarella cheese.
Spoon the beef mixture evenly into each zucchini shell.
Sprinkle the remaining mozzarella cheese and the grated Parmesan cheese on top of each stuffed zucchini boat.
Bake in the preheated oven for 20 minutes, or until the zucchini is tender and the cheese is melted and golden.
Remove from the oven and let cool for 5 minutes before serving.