Step 1: Preheat your oven to 375°F (190°C). Lightly grease a baking dish large enough to hold your zucchini halves snugly.
Step 2: Prepare the zucchini. Wash the zucchini, trim the ends, then slice each zucchini lengthwise. Using a spoon, carefully scoop out the pulp and seeds, leaving a 1/4-inch shell. Chop the scooped-out zucchini pulp finely and set aside.
Step 3: Cook the filling. In a large skillet over medium-high heat, brown the ground meat until no longer pink. Drain any excess fat. Add the chopped onion and minced garlic to the skillet and cook until softened, about 3-5 minutes.
Step 4: Add the chopped zucchini pulp, diced tomatoes (undrained), marinara sauce, Italian seasoning, salt, and black pepper. Stir well and bring to a simmer. Reduce heat to low, cover, and cook for 5-7 minutes, allowing the flavors to meld.
Step 5: Stir in 1/2 cup of the shredded mozzarella cheese and 1/4 cup of the grated Parmesan cheese into the filling mixture until just combined.
Step 6: Spoon the cooked filling generously into each zucchini boat. Arrange the filled zucchini in the prepared baking dish.
Step 7: Sprinkle the remaining 1 cup of shredded mozzarella cheese and 1/4 cup of grated Parmesan cheese evenly over the tops of the stuffed zucchini.
Step 8: Bake for 25-30 minutes, or until the zucchini is tender and the cheese is melted, bubbly, and lightly golden brown. If desired, briefly broil for 1-2 minutes at the end for extra browning, watching carefully.
Step 9: Garnish with fresh chopped parsley, if desired, and serve hot.