Combine the lemon zest, lemon juice, beaten eggs, chopped butter, and sugar in a large heatproof bowl.
Create a bain-marie by placing the bowl over a saucepan that is one-third full of gently simmering water. Ensure the bottom of the bowl does not touch the water.
Keep the heat low and whisk continuously as the butter melts to fully incorporate it.
Once combined, reduce whisking to every 10 minutes, using a spatula to scrape down the sides of the bowl to prevent the mixture from overcooking.
Cook for 45 minutes to 1 hour, or until the curd has thickened enough to coat the back of a spoon. To test if it's ready, place a small amount on a chilled plate; it should hold its shape.
Pass the finished curd through a fine-mesh sieve to remove the zest and any small lumps, ensuring a perfectly smooth texture.
Immediately pour the hot curd into hot, sterilised jam jars. Allow to cool completely before sealing and refrigerating. The curd will keep for up to 1 month in the fridge.
