Cook the lasagna noodles in a large pot of salted boiling water according to package directions. Drain well and pat dry.
Preheat oven to 350°F. Spray a 13 x 9-inch dish with nonstick cooking spray. Pour half of the marinara sauce into the bottom of the dish. Save the rest of the sauce for later.
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. Stir really well with a fork to combine. Add a pinch of optional nutmeg, if desired.
Spread the ricotta mixture in a thin layer on each of 12 lasagna noodles. Roll up the noodles and arrange them seam-side down in the prepared baking dish.
Pour or spoon the remaining half of the marinara sauce over top. Sprinkle with the 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.