01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish.
02 - Bring a large pot of salted water to a boil. Cook jumbo shells according to package directions until al dente. Drain and rinse under cold water to prevent sticking. Set aside.
03 - In a large mixing bowl, blend ricotta, mozzarella, Parmesan, chopped spinach, egg, garlic, oregano, salt, and pepper until thoroughly combined.
04 - Use a spoon to generously fill each cooled shell with the cheese and spinach mixture. Arrange shells closely in the prepared baking dish.
05 - Melt butter in a saucepan over medium heat. Whisk in flour to form a roux and cook, stirring constantly, for 1–2 minutes until lightly golden. Slowly whisk in warmed milk, ensuring a smooth consistency. Continue whisking until thickened, about 5 minutes. Season with nutmeg, salt, and white pepper.
06 - Evenly pour béchamel sauce over the filled shells, completely covering the pasta. Arrange black olive slices in pairs as eyes and small pieces for mouths to create ghost faces.
07 - Transfer the dish to the oven and bake for 25 minutes, until the sauce bubbles and the shells are thoroughly heated.
08 - Garnish with chopped parsley or chives if desired. Serve hot.