Preheat the oven to 375°F (190°C).
Wash the zucchinis and slice each in half lengthwise. Using a spoon, carefully scoop out the flesh from the center of each half, leaving about 1/4 inch thick shell. Set the scooped zucchini flesh aside.
Place the zucchini halves on a baking sheet lined with parchment paper, cut side up.
Heat olive oil in a large skillet over medium heat. Add the chopped onion and cook until translucent, about 3 minutes.
Add the minced garlic and cook for an additional 1 minute until fragrant.
Add the ground beef to the skillet. Cook, breaking it apart with a spatula, until browned and cooked through, about 6-8 minutes. Drain excess fat if necessary.
Chop the reserved zucchini flesh and add it to the skillet along with the diced tomato, dried Italian seasoning, salt, and black pepper. Stir to combine and cook for 3-4 minutes until the mixture is heated through and slightly softened.
Remove the skillet from heat and stir in half of the shredded mozzarella cheese.
Spoon the beef mixture evenly into each zucchini half, pressing down gently to fill them well.
Sprinkle the remaining mozzarella cheese and grated Parmesan 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.