Bring a large pot of salted water to a boil, then add the shells and cook for 8-10 minutes until al dente. Drain and set aside to cool slightly.
While the pasta cooks, rinse the spinach and chop it roughly. Sauté it in a skillet over medium heat until wilted, about 2-3 minutes. Let it cool slightly, then squeeze out excess moisture and chop finely.
In a mixing bowl, combine the ricotta cheese, chopped spinach, minced garlic, a pinch of nutmeg, salt, and pepper. Mix until smooth and well blended.
Using a spoon, carefully stuff each shell with the ricotta and spinach mixture, filling them generously but avoiding overstuffing to prevent spilling.
Spread a thin layer of marinara sauce on the bottom of a large baking dish. Arrange the stuffed shells seam-side down in a single layer over the sauce.
Pour the remaining marinara sauce evenly over the shells, then sprinkle shredded mozzarella cheese on top for a golden, bubbly finish.
Bake uncovered in a preheated oven at 180°C (350°F) for 25-30 minutes, until the cheese is melted, bubbly, and golden around the edges.
Remove from the oven and let rest for 5 minutes. Garnish with fresh basil or oregano if desired, then serve hot and enjoy this creamy, cheesy comfort dish.
