Place spinach in a microwave-safe bowl. Defrost in the microwave for about 5 minutes. Let rest. Once cooled place spinach in a double layer of paper towels and squeeze out all of the excess liquid possible.
To a large mixing bowl, add the ground turkey, spinach, milk, bread crumbs, onion soup mix, 1 1/2 cups cheese, egg, Worcestershire sauce, bell pepper, garlic powder, salt, and pepper. Mix everything together until well combined.
Spray an 8-inch x 8-inch glass baking dish with non-stick cooking spray.
Put the turkey mixture in the pan and spread it into an even layer.
Sprinkle the remaining 1/2 cup of cheese on top of the meatloaf. Pour the tomato sauce on top of the cheese, and sprinkle the diced onion on top of the tomato sauce.
Bake for 1 hour. If the top of your meatloaf begins to brown too quickly, carefully place a piece of foil over the top and secure loosely.
Remove meatloaf from the oven and let cool for 10 minutes before cutting into 6 slices.