Prep the pasta: Preheat the oven to 400F. Add the shells to a large pot of well salted boiling water and cook according to package instructions until they're al dente. Make sure to stir the shells often and keep an eye on them as they may stick together. Rinse well with cold water in the strainer.
Make the filling: Squeeze out as much excess water from the spinach as you can. Mix the ricotta, parmesan, spinach, egg, salt and pepper in a bowl. Scoop it into a large freezer bag or piping bag. Snip the tip of the piping bag or corner of the freezer bag to a 1" diameter.
Assemble the shells: Pour half the marinara in a 9 x 13 inch casserole dish. Pipe each shell with about 2 tbsp of the cheese and spinach mixture and place in the casserole dish over the sauce. Pour the remaining marinara sauce over the shells and top with mozzarella. Bake for 20 to 25 minutes until bubbly, then broil for 2-5 minutes or until the cheese browns.
Finish the dish: Garnish with fresh basil and freshly cracked pepper.
Transfer leftover baked stuffed shells to airtight containers and refrigerate for 3 to 5 days.
Freeze in a tightly sealed dish for up to a month. When ready to serve, take it out of the freezer the night before and thaw it in the refrigerator. Bake according to instructions.