Preheat the oven to 400 degrees. Place 3/4 cup marinara sauce in the bottom of a 13x9" baking pan and 1/4 cup in an 8x8" pan. Set aside.
Using a melon baller, carefully scoop out the flesh of each zucchini half, leaving 1/4" border on the bottom and along the sides. Chop the flesh or process briefly in your food processor and set aside (optional).
Heat 1 tablespoon olive oil in a large saucepan. Add the onion and cook 3 minutes. Add the garlic and saute 30 seconds more.
Add the chicken and chopped zucchini flesh along with salt and pepper to taste, and break up the chicken with the back of a spoon. Cook 3-5 minutes or until the chicken is cooked through. Drain the majority of the excess liquid carefully.
Add the remaining marinara sauce and stir to combine. Simmer over medium heat for 5-10 minutes.
Place the zucchini halves in the prepared pans. Fill generously with the meat the mixture and cover both dishes with foil. Bake 20-25 minutes or until the zucchini is tender.
Top the zucchini boats with the parmesan and mozzarella, and bake until melted and bubbly. Serve and enjoy!
Notes
Nutrition information is a rough estimate all marinara sauces are not the same. I advise to use one low in sugar and sodium to keep it close to these numbers.
Make sure your zucchini are the same time to ensure even cooking.
The zucchini boats and filling may be prepped ahead, and the entire meal may be assembled in advance.