Wash the blueberries in a colander.
Put the blueberries, sugar and lemon juice into a preserving pan.
Place the pan over a gentle heat to slowly dissolve the sugar crystals, stirring with a wooden spoon from time to time.
As the sugar starts to melt and the fruit warms up, take a potato masher to mash the blueberries, leaving some whole if desired.
Once the sugar has completely dissolved turn up the heat and bring the pan to a rolling boil and time for 15 minutes, stirring all the while (to prevent the sugar burning on the base).
Take the pan off the heat and test a few drops of the jam on a frozen saucer and place in the fridge for a minute. If when you push your finger through it the jam forms a crinkle and is gel like then it’s ready. Otherwise boil for another couple of minutes and repeat process until ready.
Pot up the jam in a sterilised hot jar using a small ladle or jam funnel.
Screw on lid immediately.
See notes for storage instructions.