Put the sugar and finely grated lemon rind into a mixing bowl and using a wooden spoon, crush together so that sugar takes on the flavouring oils from the lemon. Add the butter and lemon juice. Set over a saucepan one quarter filled with simmering water until the butter has melted and the sugar has dissolved.
Break up the eggs with a fork and strain into the bowl. Cook gently, stirring frequently, until thick and creamy. This will take 20-25 minutes if yolks only_are used or 10-15 minutes if whole eggs are used. Pour at once into small jars and seal while hot. Home-made lemon curd tastes better than anything you can buy, but make it in small quantities because it’s not something that keeps well.
Variation: Make a luscious lemon filling for sponges by folding 4 tablespoons lemon curd into 1/4 pint (150ml) lightly whipped double cream especially suitable for jam and cream sponge.