01 - Preheat your oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish.
02 - Boil jumbo pasta shells in generously salted water until tender yet firm to the bite (al dente), approximately 9 minutes. Drain thoroughly and rinse with cold water to halt the cooking process. Set aside.
03 - In a skillet, warm the olive oil over medium heat. Sauté the minced garlic for 1 minute until fragrant. If using fresh spinach, add it now and cook until just wilted. Skip this step if using thawed frozen spinach.
04 - In a large mixing bowl, combine the ricotta cheese, ½ cup of the mozzarella cheese, grated Parmesan, prepared spinach, whole egg, ground nutmeg, dried Italian herbs, salt, and black pepper. Blend until smooth and well incorporated.
05 - Spread 1 cup of the marinara sauce evenly across the bottom of the prepared baking dish.
06 - Using a spoon, carefully fill each cooked pasta shell with approximately 2 tablespoons of the ricotta-spinach mixture. Place the filled shells, open side facing up, in an organized layer within the baking dish.
07 - Pour the remaining marinara sauce uniformly over the arranged shells. Sprinkle the reserved ½ cup of mozzarella cheese over the top.
08 - Cover the baking dish securely with aluminum foil. Bake for 25 minutes. Then, remove the foil and continue baking for an additional 10 minutes, or until the cheese is attractively bubbly and golden brown.
09 - Allow the baked shells to rest for 5 minutes before serving to allow the flavors to meld and the dish to set slightly.