Preheat the oven on to 200⁰C (Gas Mark 6), then put a baking sheet in the oven to heat up and butter all over the insides of a souffle dish (approximately 16cm diameter, or you can use any high-sided ovenproof baking dish). Pour the milk into a pan and heat until almost boiling. In another large saucepan, melt the butter over a low-medium heat, then gradually add the flour and cook, stirring, for one minute. Take the butter and flour pan off the heat and slowly pour in the hot milk, beating with a wooden spoon as you go. Put the pan back on a medium heat and cook for another minute or two, stirring, until you have a thick white sauce. Season to taste with the salt and pepper, stir in the mustard and cayenne and take the pan off the heat.

Carefully separate the eggs, putting the whites into a large mixing bowl. Beat the yolks lightly, then beat them into the pan of sauce, together with most of the cheese – save a tablespoon or so for topping the soufflé. Whisk the egg whites until you have stiff but not dry peaks (don't overdo it). Fold the whites gently into the cheesy sauce, then pour the mixture into the greased dish. Scatter over the reserved cheese, then sit the dish on the preheated baking sheet and bake in the hot oven for about 20 minutes, until risen and golden brown. Serve straight away.