Begin by boiling a large pot of salted water.

Cook the conchiglioni pasta according to package instructions until al dente.

In a mixing bowl, combine the ricotta, mozzarella, Parmesan, chopped spinach, and beaten egg.

Once the pasta shells are cool enough to handle, use a spoon to fill each shell with the cheese mixture.

Preheat your oven to 375°F (190°C).

Spread a layer of marinara sauce on the bottom of a baking dish.

Arrange the stuffed shells in the baking dish, open side up.

Pour the remaining marinara sauce over the shells and sprinkle with extra mozzarella.

Bake the dish for about 30 minutes or until the cheese is bubbly and golden.

Let the dish rest for a few minutes before serving.