Preheat oven to 400 degrees.
Cut zucchini in half lengthwise. Place on a microwave-safe plate and heat in microwave for 2 minutes, to soften a little. Using a spoon or melon baller, scoop out flesh. Be sure to leave ¼ inch thick inside zucchini boats. Chop scooped out zucchini in small pieces. Set aside.
Place ½ cup sauce in bottom of a 9 x 9-inch baking dish. Place zucchini boats cut side up in pan.
In a large, nonstick pan, add ground beef, chopped and scooped out zucchini filling, onions, and garlic. Cook until beef is browned. Pour into a colander and drain any fat. Add beef mixture back to pan. Stir in ½ cup sauce. Season with Italian seasonings and a little salt and pepper. Mix well. Continue to cook for about 2 minutes.
To fill zucchini boats: To each, add a rounded ⅓ cup beef filling, press down to set. Top each with 2 tablespoons marinara sauce. Sprinkle each with 1 teaspoon Parmesan cheese. Repeat for remaining zucchini boats. Cover with foil and bake 35 minutes until zucchini is cooked and soft.
If desired, top each with 1 additional teaspoon of Parmesan cheese and serve. Be sure to use a spatula to scoop out each zucchini boat to place on each plate. Add some sauce to each from the bottom of pan.