Preheat your oven to 375°F (190°C).
Boil a large pot of salted water. Add the jumbo pasta shells and cook until al dente, about 9-10 minutes. Drain and set aside.
In a mixing bowl, combine the ricotta cheese, chopped spinach, half of the mozzarella cheese, Parmesan cheese, egg, minced garlic, salt, black pepper, and Italian seasoning. Mix well until fully combined.
Spread about 1 cup of marinara sauce evenly on the bottom of the baking dish.
Stuff each cooked pasta shell with the spinach and ricotta mixture, and place them in the baking dish on top of the marinara sauce.
Pour the remaining marinara sauce over the stuffed shells, then sprinkle the rest of the mozzarella cheese on top.
Cover the baking dish with aluminum foil and bake in the preheated oven for 15 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.