Place the lemon zest and cornflour in a non-reactive pan (such as stainless steel). Gradually whisk in the water to make a smooth paste then whisk in the lemon juice and sugar.
Place over a medium-low heat and stir until the sugar has dissolved. Continue to cook the curd whilst stirring until it becomes very thick.
Pour the hot lemon curd into a couple of clean (preferably sterilized) jars. Seal them and place in the fridge overnight to set before serving. Store in the refrigerator and use within a couple of weeks.