Preheat the oven to 400 degrees Fahrenheit and lightly grease a 9x13 inch baking dish.
Slice the zucchinis in half lengthwise and use a spoon or melon baller to scoop out the center flesh, leaving about a quarter-inch shell to form boats. Place the hollowed zucchini halves into the prepared baking dish.
Heat the olive oil in a large skillet over medium-high heat. Add the diced onion and minced garlic, sautéing for 3 to 4 minutes until they are soft and fragrant.
Add the ground beef to the skillet, breaking it apart with a spatula or wooden spoon. Cook until the meat is fully browned and no pink remains, then carefully drain any excess grease.
Stir the marinara sauce, Italian seasoning, salt, and black pepper into the beef. Let the mixture simmer for 5 minutes, allowing the flavors to meld and the sauce to thicken slightly.
Spoon the seasoned beef filling evenly into each zucchini boat. Sprinkle the shredded mozzarella cheese generously over the top of each boat, then bake uncovered for 20 to 25 minutes until the zucchini is tender and the cheese is golden and bubbly. Garnish with fresh chopped parsley before serving.