Cook the pasta shells according to package instructions until al dente. Drain and set aside.
In a mixing bowl, combine ricotta, chopped spinach, parmesan, garlic, Italian seasoning, and egg. Mix until smooth.
Fill each cooked pasta shell with a generous amount of ricotta mixture.
Spread half of the marinara sauce in the bottom of a baking dish and arrange the stuffed shells on top.
Pour the remaining marinara sauce over the shells and sprinkle with shredded mozzarella.
Cover and bake at 375°F (190°C) for 25 minutes. Uncover and bake for another 10 minutes until the cheese is golden and bubbly.
Let cool slightly before serving. Garnish with fresh basil and extra parmesan if desired.