Preheat the oven to 350°F. Boil a large pot of salted water and cook the jumbo pasta shells until al dente, about 8-10 minutes. Drain and set aside.
Heat the olive oil in a large skillet over medium heat. Add minced garlic and sauté until fragrant, about 1 minute.
Add the fresh baby spinach and cook until wilted, about 3-4 minutes. Remove from heat and let cool slightly.
In a large mixing bowl, combine ricotta, egg, Italian seasoning, salt, and pepper. Add the cooked spinach mixture and stir until combined.
Add the mozzarella and parmesan cheeses to the mixture and stir until smooth.
Pour the marinara sauce in the bottom of a 9x13 baking dish.
Take each cooked pasta shell and stuff it with the spinach-ricotta mixture. Arrange the stuffed shells in the baking dish.
Cover the dish with aluminum foil and bake in the preheated oven for 20 minutes. Remove the foil and add extra mozzarella cheese. Bake for an additional 5-7 minutes, or until the cheese is bubbly and golden.