Preheat the oven to 375°F (190°C).
Wash the zucchinis and slice them in half lengthwise.
Using a spoon, carefully scoop out the flesh from each zucchini half, leaving about 1/4 inch thickness to create boats. Reserve the scooped flesh and chop it finely.
Place the zucchini boats on a baking sheet lined with parchment paper and brush them with 1 tablespoon of olive oil. Set aside.
Heat the remaining 1 tablespoon of olive oil in a large skillet over medium heat.
Add the diced onion and minced garlic to the skillet and sauté for 2-3 minutes until softened.
Add the ground beef to the skillet and cook until browned, breaking it apart with a spoon as it cooks, about 5-7 minutes.
Stir in the chopped zucchini flesh, diced tomatoes, tomato sauce, dried Italian seasoning, salt, and black pepper. Cook for another 5 minutes until the mixture is heated through and slightly thickened.
Remove the skillet from heat.
Spoon the beef mixture evenly into each zucchini boat, filling them generously.
Sprinkle the shredded mozzarella cheese evenly over the stuffed zucchinis.
Bake in the preheated oven for 15-20 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.