Begin by cooking the jumbo pasta shells according to the package instructions. Boil them in salted water until they are al dente, which typically takes around 8–10 minutes.

Drain the shells and set them aside to cool slightly while you prepare the filling.

In a large mixing bowl, combine the ricotta cheese, shredded mozzarella, grated Parmesan, and the egg. Mix until well incorporated.

Heat a tablespoon of olive oil in a skillet over medium heat. Add the minced garlic and cook for about 30 seconds until fragrant. Toss in the chopped spinach and cook until wilted, about 2–3 minutes.

Stir well to combine, and season the filling with Italian seasoning, salt, and freshly ground black pepper.

Preheat your oven to 375°F (190°C).

Spread 1 cup of marinara sauce evenly over the bottom of a 9×13-inch baking dish.

Take each cooked shell and fill it with about 2 tablespoons of the ricotta-spinach mixture.

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

Once all the shells are stuffed and in the dish, spoon the remaining marinara sauce over the top.

Cover the baking dish with aluminum foil and bake for 20 minutes. Then, remove the foil and bake for an additional 10 minutes.

Let the dish rest for a few minutes before serving to allow the flavors to meld together.