Preheat oven to 375 degrees F.
Cook pasta shells according to the directions on the box. Be sure to cook extra as some may tear or break while cooking.
While those are cooking, mix together the ricotta cheese, egg, 1 1/2 cups of shredded mozzarella cheese, and salt to taste in a medium bowl.
Lightly spray a 9x13 inch baking dish with cooking spray and pour 1 1/2 cups of marinara sauce in the bottom.
When the pasta shells are finished cooking, drain them and run cool water over them to stop the cooking and cool them down.
Gently spread each pasta shell apart and stuff them with the ricotta cheese mixture.
Once they are all stuffed and in the baking dish, pour 1 1/2 cups of marinara sauce over the top and spread it out evenly.
Sprinkle the remaining 1/2 cup of mozzarella cheese on top, cover with tinfoil, and bake for 35 minutes.
Remove tinfoil and bake another 10-15 minutes.
Rest for 5 minutes and serve.