Bring a large bottle of water to a boil. Add your pasta and cook according to the package directions. Drain the noodles and set aside.
Preheat your oven to 350 degrees. Spread 1/2 cup of your tomato sauce in the bottom of a 9x9 dish.
Add the uncooked sausage to a hot pan and cook until browned. Stir in 1 cup of your tomato sauce and set aside.
In a small bowl stir together the ricotta, 1/2 a cup of mozzarella, Parmesan, egg, and spinach. Stir the mixture in with your sausage.
Spoon a hearty amount of the filling into each shell. Place into your prepared baking dish and continue until all are filled. Smother with the remaining 1/2 cup of sauce and generously sprinkle with mozzarella.
Cover with foil and bake for 20 minutes. Uncover and continue to bake for an additional ten minutes or until the cheese is melted and bubbly.
Follow the instructions above until just before baking. Cover securely with heavy duty foil or a freezer container.
Take your stuffed shells out of the freezer the day before you plan on cooking them. Let them slightly thaw in the refrigerator.
Bake at 350 degrees, covered, for 45 minutes. Remove the foil and bake an additional 20 minutes.
*If you like a more saucy pasta, add 1 extra cup onto your shells. This is also a good idea if you plan on freezing these for later.