Preheat your oven to 375°F (190°C). Wash the zucchini and slice them in half lengthwise.

Use a spoon or a melon baller to scoop out the flesh of the zucchini, leaving about a ¼-inch shell. Chop the scooped-out flesh finely and set it aside.

Heat the olive oil in a large skillet over medium-high heat. Add the ground meat and cook, breaking it up with a spoon, until it's browned. Drain off any excess grease. Add the chopped onion and cook until softened, about 5 minutes. Stir in the minced garlic and cook for another minute until fragrant.

Add the chopped zucchini flesh, diced tomatoes (undrained), tomato paste, Italian seasoning, salt, and pepper to the skillet. Stir well to combine. Bring the mixture to a simmer and cook for about 10 minutes, or until the zucchini flesh is tender and most of the liquid has evaporated.

Remove the skillet from the heat. Stir in the breadcrumbs and Parmesan cheese. Taste and adjust the seasoning if needed. Spoon the filling evenly into the hollowed-out zucchini boats.

Place 2 tablespoon of tomato paste on top of each filled zucchini.

Arrange the filled zucchini boats in a baking dish. Sprinkle the shredded mozzarella cheese evenly over the top. Bake for 20-25 minutes, or until the zucchini is tender and the cheese is melted and bubbly.

Let the zucchini boats cool slightly before serving. Garnish with fresh parsley or basil, if desired.