in a medium sized saucepan. Remove the pan from the heat and stir in the flour until a thick paste has formed. Add the milk, a little at a time and stir constantly with a whisk. Whisk in the remaining milk and then whilst stirring continuously, bring the mixture to the boil. As the sauce starts to boil, reduce the heat and simmer for 5 minutes. Stir in the salt and pepper, grated nutmeg and mustard (optional). Remove from the heat and add the egg yolks and the grated cheese. Mix the ingredients thoroughly. In a separate bowl, whisk the 6 egg whites until stiff peaks have formed. Do not use a plastic bowl; a copper or glass bowl is best. If you do not have a copper bowl, add a pinch of cream of tarter once soft peaks have started to form. Using a metal spoon or a rubber spatula, stir in  of the egg whites into the cheese sauce base. Fold the remaining whisked egg whites into the cheese mixture. Transfer the souffl mixture into the prepared souffl dish, taking care not to overfill the dish, and bake in the hot oven for between 20 - 25 minutes or until the top is crispy and brown in colour. Preheat the oven to 400F / 200C and grease a 2 pint souffl dish with butter, then line with sugar. Place the crushed amaretti biscuits in a small bowl and pour 2 tbsp of the Amaretto liqueur over them. Set