Put the blueberries, lemon juice and zest in a small pan and bring to a boil.
Let simmer for a couple of minutes until all blueberries are soft.
Strain through a sieve and press all the juice through is. Make sure to scrape the underside of the sieve.
Return the juice to the pan, set aside and let cool.
Beat the egg yolk, egg and sugar until the mixture reaches a light-yellow colour.
Pour the egg-mixture into the blueberry lemon juice and mix well.
Bring slowly to a boil and let simmer for a couple of minutes on low heat, whisking constantly.
Let cook until the mixture starts to thicken. When the ‘sauce’ covers the back of a spoon, it’s ready.
Take the pan off the heat and beat in the butter, a small cube at a time, until all the butter has been incorporated.
Pour the curd in clean jars and put the lid on.
Let cool completely.