Step 1: Preheat your oven to 375°F (190°C). Prepare a baking dish by lightly greasing it or lining it with parchment paper.
Step 2: Wash and trim the ends of your zucchini. Carefully slice each zucchini in half lengthwise. Using a spoon, scoop out the flesh and seeds, leaving about a 1/4-inch shell. Chop the scooped-out zucchini flesh finely and set aside.
Step 3: In a large skillet, heat a tablespoon of olive oil over medium heat. Add the chopped onion and cook until softened, about 5 minutes. Add the minced garlic and cook for another minute until fragrant.
Step 4: Add the ground meat to the skillet. Break it up with a spoon and cook until browned, about 8-10 minutes. Drain any excess fat.
Step 5: Stir in the finely chopped zucchini flesh, diced tomatoes (undrained), tomato paste, Italian seasoning, salt, and black pepper. Bring the mixture to a simmer, then reduce heat and let it cook for 5-7 minutes, allowing the flavors to meld.
Step 6: Remove the skillet from the heat. Stir in half of the shredded mozzarella cheese and all of the Parmesan cheese into the meat mixture.
Step 7: Arrange the hollowed-out zucchini halves in the prepared baking dish. Season the inside of each zucchini boat lightly with salt and pepper.
Step 8: Spoon the meat filling generously into each zucchini half, mounding it slightly.
Step 9: Bake for 25-30 minutes, or until the zucchini is tender-crisp.
Step 10: Remove the dish from the oven and sprinkle the remaining mozzarella cheese over the top of each zucchini boat.
Step 11: Return to the oven and bake for another 5-10 minutes, or until the cheese is melted and bubbly and lightly golden brown.
Step 12: Garnish with fresh chopped parsley before serving. Let them cool slightly before serving.