Preheat and prep dish: Preheat the oven to 375°F and grease a 9x13 inch baking dish with oil or cooking spray so the casserole releases easily once baked.

Shred and dry zucchini: Grate the zucchini on the large holes of a box grater, then wrap the shreds in a clean kitchen towel and squeeze firmly over the sink until the liquid stops dripping.

Sauté onion and garlic: Heat olive oil in a large skillet over medium heat, add the diced onion, and cook until soft and translucent, about 4 minutes. Add the garlic and cook 30 seconds until fragrant.

Brown the turkey: Add the ground turkey to the skillet, breaking it into small crumbles with a spatula, and cook until no pink remains, about 6 to 8 minutes. Drain any excess liquid from the pan.

Season and cool filling: Remove the skillet from heat and stir in the dried zucchini, Italian seasoning, salt, pepper, and red pepper flakes. Let the mixture cool for a few minutes so it won't cook the eggs.

Mix in eggs and cheese: Stir the eggs, half the mozzarella, and the parmesan cheese into the cooled turkey mixture until evenly combined and the filling looks cohesive.

Spread into baking dish: Transfer the mixture to the prepared baking dish and press it into an even layer with a spatula so it bakes evenly.

Top with marinara: Spoon the marinara sauce evenly over the top of the turkey mixture, spreading it to the edges of the dish.

Add remaining cheese: Sprinkle the remaining mozzarella cheese over the sauce in an even layer.

Bake until bubbling: Bake uncovered for 25 to 30 minutes, until the cheese turns golden and bubbles at the edges of the dish.

Rest before slicing: Remove the casserole from the oven and let it rest for 5 to 10 minutes so the slices hold together instead of spreading.

Garnish and serve: Scatter fresh basil over the top and slice into squares to serve.