Preheat your oven to 190°C (375°F) to get it ready for baking later.
Bring a large pot of salted water to a boil, then carefully add the pasta shells. Cook until just al dente, about 10 minutes, until they are tender yet still hold their shape. Drain and set aside to cool slightly.
While the shells cook, combine ricotta, mozzarella, Parmesan, chopped spinach, and a pinch of nutmeg in a mixing bowl. Mix until the ingredients are well incorporated and the filling is smooth and creamy.
Gently stuff each shell with the cheese mixture using a spoon or piping bag, pressing gently to fill each one completely.
Arrange the stuffed shells in a baking dish, placing them close together. Spoon marinara sauce generously over the top of the shells, ensuring they are well coated, then sprinkle a little extra cheese on top for a bubbly, golden finish.
Bake uncovered in the preheated oven for 25 to 30 minutes, until the sauce is bubbling around the edges and the cheese is golden brown and melty.
Remove the dish from the oven and let it rest for about 5 minutes. This helps the filling set slightly, making it easier to serve.
Serve hot, with extra marinara on the side if desired, and enjoy your bold, cheese-stuffed creation.