01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish.
02 - Boil jumbo shells in salted water until al dente following package time. Drain and rinse with cold water; set aside.
03 - In a mixing bowl, thoroughly blend ricotta, mozzarella, Parmesan, chopped spinach, egg, garlic, oregano, salt, and black pepper.
04 - Using a spoon, fill each cooked shell with the cheese-spinach mixture and organize them snugly in the prepared baking dish.
05 - Melt butter in a saucepan over medium heat. Whisk in flour and cook until pale golden, 1-2 minutes. Gradually add warmed milk, whisking consistently, until thickened and smooth, about 5 minutes. Stir in nutmeg, salt, and white pepper.
06 - Ladle the béchamel evenly over stuffed shells to cover. Arrange black olive slices in pairs for eyes and use small pieces for mouths to create ghost faces.
07 - Transfer dish to oven and bake for 25 minutes, until sauce is bubbling and shells are thoroughly heated.
08 - Top with chopped parsley or chives for a fresh finish, if desired. Serve hot.