Preheat oven to 450'.
Beat together eggs, milk, flour, and salt until very smooth. Add some very thinly sliced apples if desired. In a heavy 12-inch skillet**, melt about 1 1/2 Tbs. butter. As soon as it is quite hot, pour in the batter and put the skillet in the oven.
After 10 minutes, lower the temperature to 350' and continue baking for another 10-15 minutes. The pancake should be light brown and crisp. (Important: During the first 10 or 15 minutes of baking, the pancake may puff up in large bubbles. If it does, pierce it thoroughly with a fork.)
While the pancake is baking, prepare the apple filling. Peel and thinly slice the apples. Sauté them in 1/4 cup butter and add 1/4 cup sugar. Season to taste with cinnamon and nutmeg. The apples should be just tender, not too soft (about 8-10 minutes on medium heat).
When the pancake is ready, slide it onto an oval platter, pour the filling over one side, and fold the other side over. Or leave it round and pile the filling in the middle. Top with a little melted butter if desired. Sprinkle with powdered sugar and serve at once, slicing pieces off crosswise.