Cook the jumbo shells in a large pot of salted boiling water according to package instructions until al dente. Drain and set aside to cool slightly.
In a large mixing bowl, combine ricotta cheese, cooked spinach, half of the mozzarella cheese, Parmesan cheese, beaten egg, minced garlic, oregano, basil, salt, and pepper. Mix until well combined.
Spoon a generous amount of the ricotta and spinach mixture into each cooked pasta shell.
Spread a thin layer of marinara sauce on the bottom of a 9x13 inch baking dish. Arrange the stuffed shells in the dish, seam side up.
Top the shells with the remaining marinara sauce and sprinkle the rest of the mozzarella cheese on top.
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.
Let the dish cool for a few minutes before serving. Garnish with fresh basil if desired.