Cook the Pasta:
Bring a large pot of salted water to a boil. Add the jumbo shells and cook for 1–2 minutes less than the package instructions. Drain and rinse with cold water to stop the cooking. Set aside.
Prepare the Filling:
In a large mixing bowl, combine ricotta cheese, 1 cup of mozzarella, Parmesan cheese, egg, Italian seasoning, spinach (if using), salt, and pepper. Mix until well combined.
Preheat the Oven:
Preheat your oven to 375°F (190°C).
Stuff the Shells:
Spoon the ricotta mixture into each shell or use a piping bag for easier filling.
Assemble the Dish:
Spread 1 cup of marinara sauce on the bottom of a 9x13-inch baking dish. Arrange the stuffed shells on top in a single layer. Spoon the remaining marinara sauce over the shells.
Bake:
Sprinkle the remaining mozzarella cheese over the shells. Cover the dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 10 minutes or until the cheese is bubbly and golden.
Serve:
Remove from the oven and let rest for 5 minutes. Garnish with fresh basil and serve warm.