Preheat oven to 400°F. Spray a 9-inch by 13-inch baking dish with cooking spray; set aside.
Squeeze all of the liquid out of the spinach. I like to use a dish towel for this, and just keep squeezing until you wring out as much water as possible. Place spinach in a large bowl. Add ricotta, 1 cup of the mozzarella, ¼ cup of the Parmesan, egg, basil, salt and garlic powder to the bowl with the spinach and stir really well with a fork to combine. Add a pinch of optional nutmeg, if desired.
Pour a very thin layer of marinara sauce into the prepared baking dish to cover bottom (about ½ cup). Arrange half of the frozen ravioli in a single layer over the sauce. Spread or dollop half of the ricotta mixture over the ravioli. Top with half of the remaining marinara.
Repeat layers once more, adding remaining ravioli, then remaining ricotta mixture, and finally remaining marinara. Sprinkle remaining 1 cup of mozzarella and remaining ¼ cup of Parmesan on top.
Cover with aluminum foil and bake for 20 minutes. Remove foil and bake, uncovered, for 20-25 more minutes or until bubbly and hot in center. Let stand 10 minutes before serving.