Cook pasta shells to al dente according to package directions. Do not overcook. Run cold water over them to cool.
Meanwhile, in a medium bowl, mix together ricotta, spinach, Parmesan, egg, salt, Italian seasoning, and pepper. Add to a zip top bag** and cut the end (or a piping bag). Use a bag clip to close it to make it easier to handle.
Add 1 cup pasta sauce to the bottom of the baking dish and spread.
Pipe ricotta mixture into shells evenly and place in the baking pan, squeezing them in to fit.
Spoon remaining pasta sauce over top, and sprinkle with mozzarella cheese.
Bake for 30 minutes, or until bubbling and cheese is lightly browned.
*I suggest cooking a couple extra shells in case they break. However, stuffing broken ones will work and taste just as great!**I use a bag clip to keep the bag sealed well and easy to handle