Preheat the oven to 350°F/180°C.
Bring a large pot of water to a boil and cook the pasta according to the directions. Drain and cool the jumbo shells.
While the pasta is cooling, assemble the filling. Place the spinach in a microwave-safe container and microwave for 1 minute on High. Allow the spinach to cool slightly and squeeze out the excess water in several layers of paper towels or in a clean kitchen towel. Chop the spinach.
In a mixing bowl, combine the ricotta cheese, egg, basil, pepper, salt, 1/2 c. of the Parmesan cheese and spinach.
Using a paper towel or mister, apply a thin coat of vegetable oil to the inside of a 9x13-in. rectangular baker.
Pour 1 3/4 c. of marinara sauce into the bottom of the pan. Fill the jumbo shells with the cheese filling and place them in the pan. Pour the remaining marinara sauce over the filled shells.
Sprinkle with remaining 1/4 c. of Parmesan cheese and shredded mozzarella.
Cover the pan with aluminum foil and bake on the middle rack in the oven for 20 - 25 minutes.
Uncover the pan and continue to bake for 10 minutes or until the cheese is melted and sauce is bubbly. Serve.