Preheat oven to 375°F.
Bring a large pot of salted water to a boil. Cook shells until very al dente, 6–7 minutes.
Drain and rinse with cool water. Toss shells with olive oil to prevent sticking.
Combine ricotta, 1½ cups mozzarella, ½ cup Parmesan, egg, parsley, garlic, salt, pepper, and Italian seasoning until creamy.
Spread 1 cup marinara over the bottom of a 9x13-inch baking dish.
Fill each shell with the cheese mixture using a spoon or piping bag.
Arrange stuffed shells in a single layer in the dish.
Pour remaining marinara over shells. Sprinkle with remaining mozzarella and Parmesan.
Cover tightly with foil and bake 25 minutes. Uncover and bake 10–15 minutes more until bubbling and lightly browned. Rest 10 minutes; garnish with parsley.