In a heatproof glass bowl, whisk up 3 eggs, 200g (7oz) caster sugar, the juice of 4 unwaxed lemons (125ml/4fl oz) and 2 heaped tbsps of grated lemon zest.
Place the bowl over a small saucepan filled with 2.5cm (1in) of simmering water. Make sure the water doesn’t touch the base of the bowl.
Stir the mixture until it is hot, then slowly add 125g (4oz) of cubed unsalted butter. Whisk until all the butter has been incorporated.
Cook over a low heat for 10 minutes, whisking constantly. Keeping the heat low will prevent the eggs cooking too fast and forming lumps.
It’s done when the mixture becomes glossy and thickly coats the back of a spoon. Remove from the heat and let it cool just slightly.
Pour into sterilised jars (makes 500ml/ 17fl oz). While still warm, cover the tops with nonstick baking paper and seal. Keep in the fridge.