Preheat the oven to 375°F (190°C).
Wash the zucchinis and cut them in half lengthwise.
Using a spoon, carefully scoop out the flesh from the center of each zucchini half, leaving about 1/4 inch thick walls to create boats. Reserve the scooped flesh and chop it finely.
Place the zucchini boats on a baking sheet lined with parchment paper or lightly greased.
Heat 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 softened.
Add the ground beef to the skillet and cook, breaking it apart with a spoon, until browned and cooked through, about 6-8 minutes.
Stir in the chopped zucchini flesh, diced tomatoes, tomato sauce, dried oregano, dried basil, salt, and black pepper.
Cook the mixture for another 5 minutes, stirring occasionally, until heated through and slightly thickened.
Spoon the beef mixture evenly into the hollowed zucchini boats.
Sprinkle shredded mozzarella cheese evenly over the stuffed zucchinis.
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.