Preheat oven to 350 degrees F. Spray a 9 x 13-inch dish with cooking spray. Pour half of the marinara sauce into the bottom of the dish. Save the rest of the sauce for later.
Prepare pasta according to package instructions; drain and rinse under cold water.
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 ½ cups of the mozzarella, 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. Stuff each shell with about 2 tablespoons of the spinach mixture.
Arrange stuffed shells in the prepared baking dish, open side up. Pour remaining half of the marinara sauce over top. Sprinkle with remaining ½ cup of mozzarella.
Cover with foil and bake for about 30 minutes, or until heated through. Remove the foil cover during the final 5-10 minutes to allow the cheese to get a little bit crispy on top.