Grate the lemon until you have a tablespoon of lemon zest. Put that aside.

Then, cut the lemon in half and squeeze it until you have 1/2 cup of lemon juice. Put that aside too.

In a double-boiler, mix eggs and sugar. Then, add the butter, lemon juice and the lemon zest. Stir over simmering water until you get a thick consistensy.