Preheat the oven to 350ºF (177ºC).
Cut the zucchini in half lengthwise and scoop the seeds out using a spoon. Spread half of the tomato sauce on the bottom of a baking dish. Place the hollowed-out zucchini cut side up on top of the tomato sauce. Set aside.
Heat the oil in a frying pan or skillet over medium-high heat. Add the turkey and cook until browned, breaking it into small pieces as it cooks.
Add the garlic, Italian seasoning, salt, and red pepper flakes to the turkey and stir to combine. Add the bell pepper and spinach and continue to cook until the spinach has wilted. Pour the remaining tomato sauce into the turkey mixture and stir to combine. Season with additional salt, if necessary.
Spoon the turkey mixture into the hollowed out zucchini boats.
Top with shredded cheese (optional)
Cover the baking dish with a lid or foil and bake for 20 to 25 minutes or until the zucchini is tender.
Divide zucchini boats between plates and enjoy!