01 - Preheat oven to 375°F.
02 - Slice zucchinis in half lengthwise. Scoop out pulp with a spoon, leaving 1/2-inch thick shells. Finely chop the pulp and set aside.
03 - Heat olive oil in a large skillet over medium heat. Add onion and sauté for 2-3 minutes until softened. Add garlic and cook for 30 seconds until fragrant.
04 - Add ground beef to skillet. Cook, breaking up with a spoon, until browned, about 5-6 minutes. Drain excess fat if necessary.
05 - Stir in chopped zucchini pulp, diced tomato, tomato paste, oregano, basil, salt, and pepper. Cook for 4-5 minutes until well combined and most liquid is absorbed.
06 - Remove from heat. Stir in breadcrumbs, Parmesan cheese, and fresh parsley until evenly distributed.
07 - Arrange zucchini halves in a baking dish. Fill each with beef mixture, pressing lightly to pack filling securely.
08 - Sprinkle mozzarella cheese evenly over stuffed zucchini tops.
09 - Cover baking dish with foil. Bake for 20 minutes.
10 - Remove foil and bake for an additional 10-15 minutes until cheese is golden and bubbly.
11 - Garnish with additional fresh parsley and serve hot.