How to Make It
Preheat the oven to 400°. Butter an 8-by-12-inch glass or ceramic baking dish. In a medium saucepan, melt the butter over moderate heat. Add the flour and whisk until blended and bubbling. Gradually whisk in the milk and bring to a boil, whisking constantly; simmer for 3 minutes. Gradually whisk in the cheese until melted. Add mustard if desired and season generously with salt and pepper. Whisk in the egg yolks.
In a bowl, beat the egg whites until firm. Whisk half of the whites into the soufflé base, then fold the base into the remaining whites. Pour the mixture into the baking dish and sprinkle with Parmesan. Bake for 20 minutes, or until browned and puffed. Serve hot.