Zest and squeeze lemons. Strain the juice briefly to remove any seeds.
Place the lemon zest, sugar, and butter in a bowl. You are making this recipe au bain marie, so place the bowl over a saucepan of boiling water. Do not turn the heat up too high. Let the ingredients melt in the bowl and stir gently until smooth.
When all the butter is melted, add the lemon juice and stir. Add the eggs and stir again. Now it is a matter of patience. Check from time to time to make sure there is enough water in the pan.
Continue to stir gently in the bowl as the lemon curd continues to thicken. When the curd is the consistency of yogurt, it is ready. This takes about 20 minutes.
Place the lemon curd in a bowl to cool. As the curd cools, it will continue to thicken to the proper final thickness.
How to store
When the lemon curd has cooled completely, place it in a jar and store in the refrigerator for up to two weeks. If you boil the jars first, you can store the curd in a cool, dark place outside the refrigerator for up to a year.