Lightly grease 4 ovenproof dishes or teacups, then stand them in a roasting tin.
Place the egg yolk and caster sugar in a large bowl and whisk together, using an electric mixer, until creamy. Whisk in the flour, lemon rind and juice until smooth, then gradually whisk in the milk.
In a separate clean bowl, whisk the egg white until soft peaks form. Lightly fold into the lemon mixture and spoon into the cups or dishes. Pour boiling water into the roasting tin to come halfway up the sides of the cups or dishes and bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 15–20 minutes until risen and golden. The sauce will separate during cooking underneath the fluffy tops.