Boil the water: Bring a large pot of salted water to a boil.

Cook the shells: Heat the oven to 375 F, add the shell pasta to the boiling water in the pot, and cook the pasta until just flexible, then drain and cool it on a sheet pan.

Mix the filling: Stir the ricotta with the spinach, egg, 1/4 cup of the Parmesan, 1 teaspoon of the salt, and the black pepper.

Sauce the dish: Spread 1 cup of the marinara over the bottom of a baking dish.

Stuff the shells: Fill each shell with a spoonful of the ricotta mixture and nestle it in the sauce.

Top the pan: Spoon the remaining marinara over the shells, then scatter over the mozzarella and the remaining Parmesan.

Bake until hot: Cover the dish with foil and bake for 25 minutes, then uncover and bake for 10 minutes more until bubbling.

Rest and finish: Let the shells rest for 5 minutes, then top with parsley if using and serve.