Mix the flour, sugar and salt. Stir in the lemon rind, juice, egg yolk and milk. Beat the egg white until it forms stiff peaks. Fold it into the juice and egg mixture.
Pour into 4 3" custard cups or a small round baking dish. Set the custard cups or baking dish in pan of hot water 1" deep. Bake for 50 minutes.
Serve warm or cold, with or without whipped cream.