Preheat the oven to 190°C. Butter a 1.5 litre soufflé dish. Melt the butter in a medium saucepan over medium heat, stir in the flour and cook on low heat for 1 minute. Gradually add the milk and bring to the boil, stirring all the time until very thick and smooth. Remove the saucepan from the heat and stir in the grated Cheddar cheese, salt, pepper and mustard or nutmeg. Leave to cool slightly, then beat in the egg yolks, one at a time.
Whisk the egg whites in a clean, grease-free bowl until they are stiff and glossy, but not dry. Add 2–3 tablespoons of the whisked whites to the cheese sauce and stir in to thin the sauce to a consistency similar to that of the whisked egg whites. Gently fold the remaining whisked egg whites into the sauce.
Pour the mixture into the prepared soufflé dish. Sprinkle with Parmesan cheese and bake in the centre of the oven for about 30 minutes, or until it is well risen, golden and just set. Do not open the door during cooking time. Serve the soufflé immediately, either as a starter or with a salad for a light lunch or supper.