Wash the apples, then chop them into small chunks – there is no need to peel or core them – and put into a preserving pan.
Wash the blueberries and place into the preserving pan with the apples and apple juice.
Add the orange zest.
Bring to the boil, then reduce the heat and simmer for 20 minutes or until the apples have collapsed, stirring occasionally.
Strain the mixture through a fine sieve, then return the blueberry and apple mixture to the pan.
Add the sugar and return to the heat. Stir constantly until the sugar is dissolved.
Continue to cook until a thick, creamy consistency is achieved – this can take up to an hour. Stir from time to time.
Pour into sterilised jars. Cover with a lid and label.
Store the butter in the fridge and eat within a week.