Preheat oven to 375°F. Boil shells until al dente; drain and cool.
Heat oil in a skillet. Cook turkey until no pink remains. Season with salt, pepper, and Italian seasoning.
Stir in chopped spinach until wilted. Remove from heat and let cool slightly.
Mix turkey-spinach with ricotta, egg, Parmesan, and 1 cup mozzarella.
Spread 1 cup marinara in a baking dish. Stuff shells with filling and place in dish.
Pour remaining marinara over shells and sprinkle with 1/2 cup mozzarella.
Cover with foil and bake 20 minutes. Remove foil and bake 5 more minutes until cheese browns.