Wash the blueberries thoroughly under cold water and drain them well. Sort through them to remove any spoiled ones. Prepare a little more than the final quantity needed for the recipe.

Place the blueberries in a large pot or jam pan. Add the sugar, water, and lemon juice, and mix everything together well.

Let the mixture macerate for at least 2 hours at room temperature, or preferably overnight in the refrigerator. This step allows the blueberries to release their juice and the sugar to begin dissolving, which helps speed up the cooking process.

Stir the blueberries and the syrup that has formed, then place the pot over high heat.

Once the mixture comes to a boil, add a small knob of butter to stop the foam. Stir regularly.

Cook for 13 to 15 minutes from the start of boiling. After 5 minutes, lower the heat to medium and continue cooking.

You can use a potato masher to crush the fruits more, depending on the desired consistency of the jam.

During cooking, foam may form on the surface despite the knob of butter. Remove it towards the end of the cooking process using a skimmer.

Once cooking is finished, remove the jam from the heat and, while still hot, pour it into the sterilized jars using a ladle and funnel, if possible.

Fill the jars to about 1 cm from the top. Wipe the rims of the jars if necessary, then seal them immediately with the lids.

Turn the jars upside down on a cloth and let them cool completely. After an hour, check that the lids are still tightly sealed.

Once cooled, store the jars in a dry place away from light. Once opened, keep the jar in the refrigerator and consume it within the following weeks.