Place a metal bowl above a saucepan of simmering water.
2.
In this bowl, whisk together eggs yolks and sugar until smooth.
3.
Mix in lemon juice and zest until combined.
4.
Stir the mixture over the heat, scraping the sides and bottom of the bowl, until it thickens - thick enough to coat the back of a spoon. This should take around 7 minutes.
5.
Pour into jars and seal once cooled. Store in the fridge.
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