Begin by boiling a large pot of salted water.

In a mixing bowl, combine ricotta cheese, mozzarella cheese, Parmesan cheese, egg, and Italian seasoning.

While the pasta cooks, preheat your oven to 375°F (190°C).

Once the pasta is al dente, drain it in a colander and let it cool slightly.

Take each shell and gently fill it with the cheese mixture.

Spread a thin layer of marinara sauce on the bottom of an oven-safe dish.

Place the filled shells in the baking dish, open side up, and cover them with the remaining marinara sauce.

Bake in the preheated oven for about 25 minutes until bubbly and golden.