Preheat the oven to 350 degrees F. Lightly coat a 13- by 11-inch baking dish with cooking spray. Spread ⅓ cup of the tomato sauce on the bottom of the dish.
In a large bowl, combine the ground turkey, zucchini, grated onion, garlic, breadcrumbs, ¼ cup parsley, egg, salt and pepper. Stir to combine.
Using 1 ½ tablespoons per meatball (a large cookie scoop works well here), form meatballs by rolling between the palms of your hands. Place the meatballs, in a single layer, in the prepared baking dish.
Cover the meatballs with the rest of the tomato sauce and sprinkle with the feta cheese.
Bake until the meatballs are cooked through, about 25 minutes.
Garnish with the remaining 2 tablespoons of the parsley.
Heat the olive oil in a large nonstick skillet set over medium heat. Add the onion and cook, stirring occasionally, until the onion is softened, about 5 minutes.
Add the garlic, oregano and red pepper flakes, and cook for 1 minute.
Stir in the crushed tomatoes, salt and pepper. Bring to a boil, then simmer for 10 minutes.