1. Pour the milk and cream into a heavy based saucepan over a moderate heat, add the pared lemon zest and bring the milk and cream to the boil.
2. Mix the egg yolks and sugar together and gradually pour on the boiled liquid. Return the mixture to the pan. Cook the custard over a very low heat, stirring constantly, until it thickens.
3. Remove the pan from the heat and pour the sauce through a fine sieve into a clean bowl.
4. Serve the custard either hot or cold.