Get a large sauce pan, fill it with water and bring it to a boil. Add the shells, and stir often so they don’t stick together.
Take thawed out spinach and squeeze all the liquids out of it. The spinach should not have any liquids left in them. If they do your filling will become runny. Shred zucchini and squeeze all the water out of it. Dice onions and mince garlic.
In a large mixing bowl combine ricotta cheese, spinach, zucchini, onion, garlic, 2 tablespoons of Parmesan reggiano and mozzarella, Italian seasoning, salt, and red pepper flakes. Stir until it’s all blended well together.
Drain shells, and rinse well. Add ice cubes to the strainer so they can cool faster.
In a square baking dish add a cup of sauce to the bottom of the pan.
Take a shell and stuff it until it’s full (not overly filled, but enough to where the seams barely meet). Place the shell seam side up and repeat this step until all shells are filled.
If consuming right then; pour the marinara sauce over the shells making sure all of the shells are covered. Sprinkle two tablespoons each Parmesan reggiano and mozzarella over shells. *If preparing this for later reserve the other cup of sauce and cheese so that you can pour it over it once you are ready to bake.
Cover with tin-foil for 30 minutes and bake. Remove tin-foil after time is up and bake for an additional 5-10 minutes or until cheese is golden brown!