Preheat your oven to 350℉.
In a large pot of salted water cook the pasta until al dente. Drain and set aside.
In a mixing bowl, combine the ricotta, shredded mozzarella, parmesan, spinach, pesto, egg, salt, and pepper. Once mixed together, transfer to a plastic bag and snip the bottom about 1 inch deep.
To a baking dish, pour ½ cup of sauce on the bottom.
To each cooked shell, squeeze in a few tablespoons of the cheese mixture and place it open-side up in the baking dish. Repeat until the dish is full/you are out of pasta.
Pour the remaining sauce over the shells and top with a few dollops of pesto and a torn mozzarella cheese.
Bake for 30-40 minutes at 350℉ until the cheese is bubbly. Option to broil for a few minutes to get brown on the top.