Follow these instructions carefully to create a dish that will impress your family and friends. Preparation is key, so read through all the steps before you begin.

1. Preheat Oven & Prepare Baking Dish: Preheat your oven to 375°F (190°C). Spread about 1 cup of the marinara sauce evenly over the bottom of a 9×13 inch (23×33 cm) baking dish. This prevents the shells from sticking and adds a flavorful base.
2. Cook the Pasta Shells: Bring a large pot of generously salted water to a rolling boil. Add the jumbo pasta shells and cook according to package directions until al dente – typically 8-10 minutes. You want them pliable enough to stuff but firm enough not to tear easily. Be gentle when stirring to avoid breaking them. Once cooked, drain the shells carefully and rinse with cold water to stop the cooking process and prevent them from sticking together. Set them aside, perhaps lightly drizzled with a touch of olive oil to further prevent sticking.
3. Prepare the Spinach: While the pasta cooks, heat the 1 tablespoon of olive oil in a large skillet or pan over medium heat. Add the minced garlic and sauté for about 30 seconds until fragrant, being careful not to burn it. Add the fresh spinach to the skillet (it might seem like a lot, but it will wilt down significantly). Cook, stirring frequently, until the spinach is completely wilted, about 3-5 minutes. Remove from heat.
4. Squeeze the Spinach Dry: This is a crucial step! Transfer the wilted spinach to a clean kitchen towel or several layers of paper towels. Once cool enough to handle, squeeze out as much excess moisture as possible. Excess water will make your filling runny. Chop the squeezed spinach finely.
5. Make the Cheese Filling: In a large mixing bowl, combine the whole milk ricotta cheese, 1/2 cup of the grated Parmesan cheese, the beaten egg, the finely chopped (and squeezed dry!) spinach, the freshly grated nutmeg, 1/2 teaspoon of salt, and 1/4 teaspoon of black pepper. Mix thoroughly until all ingredients are well incorporated and the filling is smooth and consistent. Taste and adjust seasonings if necessary. Some people like a little more nutmeg, or perhaps a pinch of red pepper flakes for a tiny bit of heat.
6. Stuff the Shells: Now for the fun part! Carefully take one cooked pasta shell at a time. Using a small spoon (or a piping bag for a neater, quicker job), generously fill each shell with the ricotta-spinach mixture. Don’t be shy with the filling; you want them nicely plump.
7. Assemble the Casserole: As you stuff each shell, arrange it open-side up in the prepared baking dish, nestled into the layer of marinara sauce. Place them snugly next to each other in a single layer. You might need to gently push them together to fit them all.
8. Top with Sauce and Cheese: Once all the shells are filled and arranged, pour the remaining marinara sauce evenly over the stuffed shells. Ensure they are well-covered, but some of the shell tops can peek through. Sprinkle the shredded mozzarella cheese evenly over the sauce, followed by the remaining 1/2 cup of grated Parmesan cheese. This double layer of cheese on top will create a wonderfully golden and bubbly crust.
9. Bake to Perfection: Cover the baking dish loosely with aluminum foil. This prevents the cheese from browning too quickly before the shells are heated through. Bake in the preheated oven for 25 minutes. Then, carefully remove the foil and continue baking for another 15-20 minutes, or until the sauce is bubbly around the edges, the cheese is melted, golden brown, and slightly crispy in spots, and the filling is heated through.
10. Rest and Garnish: Once baked, remove the dish from the oven and let it rest for at least 10-15 minutes before serving. This allows the shells to set slightly, making them easier to serve and preventing the filling from oozing out too much. It also allows the molten cheese and sauce to cool just enough to avoid burning eager mouths. Just before serving, garnish with freshly chopped parsley and/or basil, if desired, for a pop of color and fresh flavor.