Preheat oven to 350 degrees.
In a saucepan melt the butter over low heat, stir in the flour, and cook the roux, stirring, for 3 minutes. Remove the pan from the heat and add the milk, heated, in a stream, whisking. Simmer the mixture, whisking occasionally, for 5 minutes, add the nutmeg and salt and pepper to taste, and transfer the mixture to a large bowl. Whisk in the egg yolks, 1 at a time, whisking well after each addition, and whisk in the Parmesan. In a bowl with an electric mixer beat the egg whites with a pinch of salt until they are frothy, add the cream of tartar, and beat the whites until they just hold stiff peaks. Stir one third of the whites into the mixture and fold in the remaining whites gently but thoroughly.
Spread the sponge batter evenly in a buttered 15 1/2 by 10 1/2-inch jelly roll pan lined with buttered and floured waxed paper and bake it for 25 minutes, or until it is golden and firm to the touch. Cover the egg sponge with a sheet of buttered waxed paper, buttered side down, and a dish towel, invert a baking sheet over the towel, and invert the egg sponge onto the baking sheet. Remove the jelly roll pan and the waxed paper carefully and trim 1/4-inch from the short sides of the egg sponge. Fill the egg sponge with the filling of your choice while it is still warm and flexible.