Preheat your oven to 375°F (190°C) and prepare a 9x13-inch baking dish by spraying it with non-stick cooking spray or lightly greasing it.

In a medium saucepan, combine the rinsed quinoa and chicken broth (or water). Bring to a boil, then lower the heat to a simmer. Cover and allow it to cook for about 15 minutes, or until all the liquid is absorbed and the quinoa is fluffy.

While the quinoa is cooking, heat a large skillet over medium heat. Add the ground turkey, breaking it apart with a spatula. Cook until it’s browned and cooked through, about 5-7 minutes.

Once the turkey is cooked, add the chopped onion and bell pepper to the skillet. Sauté until the vegetables are tender, about 3-5 minutes. Stir in the garlic powder, Italian seasoning, and crushed red pepper flakes (if using) for added flavor.

In a large bowl, combine the cooked quinoa, turkey and vegetable mixture, and marinara sauce. Mix everything well until fully incorporated.

Transfer the mixture to the prepared baking dish and spread it evenly. Top with the shredded mozzarella cheese and sprinkle olives over it, if desired.

Bake in the preheated oven for 25-30 minutes, or until the cheese is melted and bubbly, and the edges are lightly golden.

Once done, remove the casserole from the oven and let it cool for about 5-10 minutes before serving.

Garnish with fresh basil or parsley, and serve warm with a side salad or steamed vegetables.