Whisk the egg yolks and half the sugar together until pale and creamy. Place the milk, lemon zest and remaining sugar in a saucepan and bring to a boil.
Pour over egg mixture and whisk to combine. Pour the custard back into the saucepan and cook over low heat, stirring continuously until the mixture is thick enough to coat the back of a wooden spoon - do not allow the custard to boil.
Strain the custard into a bowl, add the lemon juice and cream and then cool over ice. Churn in an ice cream maker following the manufacturer's instructions.
Alternatively, pout into a plastic freezer box, cover and freeze. Stir every 30 minutes with a whisk during freezing to break up the ice crystals and give a better texture.