Butter 2 ramekins, each holding 150 ml (¼ pint), and coat the bottom and sides with grated Parmesan.
Melt the butter in a small saucepan and add the flour. Whisk to make a smooth paste, then gradually add the milk, stirring all the time, until the sauce thickens. Allow to cool a little, then beat in the Stilton and egg yolk.
Whisk the egg white in a clean bowl until it forms soft peaks. Fold into the cheese mixture, then spoon into the prepared ramekins. Cook in a preheated oven, 200°C (400°F), Gas Mark 6, for about 15 minutes until golden and risen. Serve hot with watercress salad (see below) and some crusty, seeded bread.