Preheat the oven to 375°F (190°C).
Wash the zucchinis and cut them in half lengthwise.
Using a spoon, scoop out the flesh from each zucchini half, leaving about 1/4 inch thick shell. Reserve the scooped flesh and chop it finely.
Place the zucchini halves cut side up on a baking sheet lined with parchment paper.
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 until browned, breaking it apart with a spoon, about 5-7 minutes.
Stir in the reserved chopped zucchini flesh, diced tomatoes, dried oregano, dried basil, salt, and black pepper.
Cook the mixture for another 5 minutes until heated through and slightly thickened.
Remove the skillet from heat and spoon the beef mixture evenly into each zucchini half.
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.