Preheat oven to 350°F. Spray a 9 x13" dish with cooking-spray. Then pour half of this marinara-sauce to the bottom of the dish. Save the remainder of the sauce for later.
Meanwhile, prepare the pasta according to package directions then drain and rinse under cold-water.
Squeeze All the liquid from the spinach. You can use a dish-towe and just keep grinding till you squeezing as much water as you can. Then put greens in a large-bowl.
Add the ricotta, 1 ½ cups of the mozzarella, parmesan, egg, garlic powder, basil and salt, into the bowl with the spinach and stir very well with a fork to blend.
Add a pinch of optional nutmeg, if desired. Stuff each shell with about 2 tbsp of the spinach mix.
Arrange stuffed-shells in the prepared baking-dish, then open side-up. Pour remaining portion of the marinara-sauce on top and sprinkle with remaining ½ cup of mozzarella.
Cover with foil and bake for 30 minutes, or till heated through.
Remove the foil cover during 5-10 minutes to permit the cheese to just a bit crispy on top.