In a large pot of salted boiling water, cook the jumbo pasta shells until al dente. Drain them and set aside.

In a skillet over medium heat, heat the olive oil. Add the minced garlic and sauté until fragrant.

Add the chopped spinach to the skillet and cook until wilted. Season with salt and pepper, then remove from heat and let cool slightly.

In a mixing bowl, combine the ricotta cheese, half of the mozzarella, half of the parmesan, the egg, and nutmeg (if using).

Add the cooked spinach mixture to the filling and mix until well combined. Season with salt and pepper to taste.

Spread 1 cup of marinara sauce evenly on the bottom of a baking dish.

Fill each cooked pasta shell with the ricotta-spinach mixture and place them in the baking dish.

Pour the remaining marinara sauce over the stuffed shells.

Sprinkle the remaining mozzarella and parmesan cheeses on top.

Cover the baking dish with foil and bake in the preheated oven for 25 minutes.

Remove the foil and bake for an additional 10 minutes, or until the cheese is melted and bubbly.

Garnish with freshly chopped basil or parsley before serving.