Preheat the oven to 180C/350F. Line a large baking sheet with parchment paper and spray with cooking spray.
Slice the zucchinis in half and scoop out the innards of the zucchini, chop it finely, and set that aside.
Heat oil in a non-stick pan and place it over medium heat. Once hot, add the garlic and cook until fragrant. Add the onion and cook for a further minute. Add the ground turkey and cook for 4-5 minutes, until no longer pink. Add the marinara sauce and Italian seasonings, along with the remaining chopped zucchini. Simmer everything for 8-10 minutes.
Distribute the filling mixture amongst the zucchini boats. Sprinkle generously with the cheese blend and bake for 15-20 minutes, or until the cheese is bubbling and lightly golden brown.
TO STORE: Leftovers can be stored in the refrigerator, covered, for up to five days. TO FREEZE: You can freeze zucchini boats, but they will become mushy when thawed if not frozen properly. To avoid making them soft, put the cooked zucchini boats in freezer-safe airtight containers and then keep them in the freezer for up to 2 months.