Preheat the oven to 375F.

Make the sauce by combining the canned tomato sauce with the oregano and garlic powder. Set aside.

Next, make the spinach filling by placing the spinach into a separate bowl together with the ricotta, beaten egg, shredded Parmesan, salt, and a LOT of black pepper.

Spoon a little of the tomato sauce over the base of a baking dish.

Add four slices of chicken over the sauce. Cover the chicken with a third of the spinach mixture, then cover that with a quarter of the remaining tomato sauce. 

Repeat two more times, then add a final layer of chicken, followed by the remaining quarter of tomato sauce.

Cover and bake for 20 minutes, then uncover and sprinkle the shredded cheese over the top.

Return to the oven (uncovered) for another 8-10 minutes or until the cheese has melted and the sauce is bubbling.

Let rest for five minutes before serving.