Prepare the pasta shells according to the package instructions, but remove them from heat 1 minute early. Drain and set aside.
Heat olive oil over medium heat. Add spinach and cook until wilted. Add garlic and cook for an additional minute. Remove from heat and set aside.
In a mixing bowl, combine ricotta cheese, eggs, Parmesan, salt, pepper, parsley, and basil. Add the cooled spinach to the mixture. Transfer the ricotta filling to a piping bag or a plastic bag with one corner cut off.
Spread a thin layer of marinara sauce at the bottom of a casserole dish. Fill each pasta shell with the ricotta mixture and arrange them in the dish.
Top the shells with more marinara sauce, followed by mozzarella and extra Parmesan cheese.
Cover the dish with foil and bake for 30 minutes. Remove the foil and continue baking for another 5 to 10 minutes, until the cheese is melted and bubbly. Allow the dish to rest for 10 minutes before serving.