Put the grated lemon zest and sugar into a large bowl

In a large measuring jug, whisk the lemon juice together with the eggs

Pour the lemon juice & egg mixture over the zest & sugar mixture

Add the butter cut into small cubes and put the bowl over a pan of just simmering water

Stir frequently until thickened - about 30 minutes

Decant into sterilised jars before screwing the lids down tightly