Preheat the oven to 375°F (190°C).
Wash the zucchinis and slice each in half lengthwise.
Using a spoon, scoop out the flesh from each zucchini half, leaving about 1/4 inch thick shell. Chop the scooped-out flesh and set aside.
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 chopped zucchini flesh, diced tomatoes, dried Italian seasoning, salt, and black pepper.
Cook the mixture for another 5 minutes until the zucchini is tender and the flavors are combined.
Remove the skillet from heat and spoon the beef mixture evenly into each zucchini half.
Sprinkle the shredded mozzarella and grated Parmesan cheese evenly over the stuffed zucchinis.
Bake in the preheated oven for 15-18 minutes until the zucchini is tender and the cheese is melted and golden.
Remove from oven and let cool for 5 minutes before serving.