You will know that you have cooked the mixture enough when it begins to coat the back of the spoon. I find that this process takes 7 – 8 minutes.
If you want to test the curd, remove the saucepan from the heat and place a spoonful of curd onto a saucer. As the curd cools on the saucer, you will notice it thicken and it shouldn’t run if the saucer is tilted.
If the curd does not show signs of thickening return it to the heat and continue stirring for another minute or two.
Once you are happy that the curd is thickening, remove it from the heat and stir in the lemon zest.
Transfer to a heat proof container or sterile jars.
Store lemon curd in the fridge. If in sterile jars it should keep unopened for 2 -3 months, once opened use within 2 weeks.
If you haven’t put your lemon curd in jars, but don’t think it will be used within 2 weeks, pop it into the freezer in an airtight container. It should last at least 6 months in the freezer.