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Put the lemon zest, juice, sugar and butter in a heatproof bowl over a pan of simmering water. Stir occasionally until the butter has melted. Then, using a small whisk or fork, stir in the beaten egg. Keep gently whisking the mixture over the heat for around 10 mins until thickened like custard.
Pour the cooked curd through a sieve into a clean jug to remove the zest, then pour into sterilised jars. Keeps in the fridge for up to 2 weeks.