WASH THE SPINACH WELL and put it in a non-aluminum pot. Cover and cook it in its own water over medium heat until wilted, about 5 minutes. Put the wilted spinach in a strainer and, with the back of a large spoon, squeeze as much liquid from the spinach as possible. If it is cool enough to handle, squeeze the water out with your hands, a small handful at a time. Chop the spinach by hand or in a food processor, season with salt, pepper and nutmeg and mix with 3 tablespoons of the butter. Use the remaining 1 tablespoon of butter to grease the gratin dish. Add the spinach, cover with Mornay Sauce (See RECIPE) and sprinkle with cheese. Twenty minutes before serving, place in a preheated 375F oven. Bake until browned and bubbly.