In a heavy saucepan cook the sugar, butter, eggs and lime juice over a moderately low heat. Whisk frequently, until it is thick enough to hold marks of whisk, about 12-15 minutes.
Immediately pour the curd through a sieve into a bowl. Stir in the zest of one lime and cool. Lime curd can be made up to a week in advance, its surface covered with cling film.
