Preheat the oven to 350°F (176°C). Make sure you do this first. I always forget. This is me telling you: Don’t forget.
In a mixing bowl, combine the ricotta, spinach, egg, salt, pepper and nutmeg. Mix everything well. This will be the filling for our shells. Add the mixture to each dry pasta shell, making sure each shell is totally full of the mixture. Scrape the top of each shell so the filling is flat and not coming out of the shell.
Lay the shells in a single layer in a 3 quart (2.8 L) oven-safe baking dish. You do not need to grease the pan as the sauce will provide enough moisture for nothing to stick. Speaking of sauce, add the marinara sauce to make sure all the shells are submerged and there is enough sauce to cover the top. I know it looks like a lot, but remember our shells are not cooked, and this sauce is going to cook them.
Cover the baking dish (I just use foil) and bake for 1 hour. Remove from the oven and add shredded mozzarella cheese. Sprinkle the Italian seasoning on top of the cheese. Put the dish back into the oven for 15 minutes or until the cheese is golden and bubbling. Remove and top with basil. Pro tip – have garlic bread ready on the side. You will need it to sop up the sauce.