Step 1: Prepare the Ingredients

Start by finely grating the zest of all four lemons and then squeezing the juice. You should get about 150ml of lemon juice. Set aside both the zest and juice.

Step 2: Set Up a Double Boiler

In a heatproof bowl, place the sugar, butter, lemon zest, and lemon juice. Set the bowl over a saucepan of simmering water (a double boiler), ensuring the bowl doesn’t touch the water. Stir the mixture occasionally until the butter has fully melted and everything is combined.

Step 3: Whisk the Eggs

In a separate bowl, whisk the eggs and egg yolk until they are smooth. Gradually pour the beaten eggs into the lemon mixture, whisking constantly to prevent the eggs from curdling.

Step 4: Cook the Lemon Curd

Continue cooking the mixture over simmering water, stirring constantly with a wooden spoon or silicone spatula. After about 10-15 minutes, the mixture will thicken and coat the back of the spoon. You want the curd to be thick but still pourable. Be patient as the thickening process takes time.

Step 5: Strain and Store

Once the curd has thickened, remove it from the heat and strain it through a fine sieve into a clean bowl. This will remove any lumps or zest, leaving you with a silky-smooth curd. Pour the lemon curd into sterilized jars and allow it to cool completely before sealing. Store in the fridge.