1. Separate the eggs.
2. Sift the flour.
3. Beat the yolks with the caster sugar until light, pale, and creamy.
4. Whisk the milk, flour, lemon juice, and rind into the egg yolks.
5. Whisk the egg whites with the salt, adding the icing sugar gradually.
6. Continue to whisk until stiff but not dry. Fold into the lemon mixture.
7. Grease a deep 1 litre ovenproof dish and pour the mixture into it.
8. Stand the dish in a shallow pan of cold water and bake in a moderate oven at 180°C, Gas 4, for 1 hour.