Bring a large pot of water to a boil. Add a healthy pinch of salt and the shells. Cook to al dente accordingn to package directions. Do not overcook. Drain and return to the pot, and add 1 tablespoon olive oil to the shells. Toss well to keep shells from sticking. Set aside.
In a 9" skillet over medium heat, add the final tablespoon olive oil, garlic and baby spinach to the skillet. Cook until the spinach is wilted about 6 to 8 minutes. Set aside to cool slightly.
In a separate bowl add the ricotta, mozzarella, and parmesan cheeses. Stir in the egg, parsley, basil, salt, nutmeg and pepper and stir to combine. Transfer to a zip top plastic bag or piping bag to fill the shells.
Cut the corner off the zip top bag and squeeze the cheese mixture in to each shell. It's easier to fill the shells if you squeeze them slightly to open them wider.
Transfer all shells to a cookie sheet to freeze then store in an air tight container for up to 2 months.
To bake right away:
Lay 1 cup of pasta sauce in the bottom of a 9x13 baking dish. Add shells to fit the pan. It's fine if they touch. Cover with additional 1-2 cups sauce and 1 cup mozzarella cheese.
Bake covered with aluminum foil for 30-40 minutes. Uncover for the last 10 minutes so the cheese can melt.
To bake from frozen:
Follow instructions as above but bake 55-60 minutes, uncovering for the last 10 minutes. If you are making a very small batch (5 shells or less) check to see if your shells are done at 45 minutes.