Whisk sugar, water and cornstarch together until cornstarch is dissolved

Pour into a small saucepan and whisk over medium heat, stirring constantly, until it comes to a full boil.

Boil for 2 - 3 minutes, not stirring. It should be thickened and turning clear.

Remove from heat and lemon juice, zest and margarine and blend well with the whisk.

Cool the curd, then refrigerate in a covered container. It thickens as it cools.