Bring a large pot of salted water to a boil, then add the pasta shells and cook until just al dente, about 8 minutes. Drain and set aside to cool slightly.
Meanwhile, heat olive oil in a skillet over medium heat. Sauté chopped garlic and finely chopped onion until fragrant and translucent, about 2-3 minutes, filling your kitchen with a warm aroma.
In a mixing bowl, combine ricotta cheese, shredded mozzarella, chopped spinach, and a pinch of nutmeg. Season generously with salt and pepper, then fold everything together until well mixed and creamy.
Use a spoon or piping bag to fill each cooked shell generously with the cheese mixture. Place the filled shells upright in a baking dish, arranging them snugly but without overcrowding.
If desired, spoon tomato sauce over the assembled shells for added flavor and moisture. Cover the dish with foil and bake in a preheated oven at 180°C (350°F) for 20 minutes.
Remove the foil and continue baking for another 10 minutes until the cheese is bubbling, slightly browned, and the edges are crispy and golden. The kitchen will be filled with the irresistible scent of melted cheese and garlic.
Once baked, let the shells rest for 5 minutes to settle and avoid cheese sliding off. Serve hot, garnished with extra spinach or fresh herbs if desired. Enjoy the gooey, chaotic comfort of these creepy stuffed shells!
