Place 1000g blueberries into clean water and gently stir to remove any surface impurities. Drain thoroughly.

Transfer the washed blueberries to a large bowl. Add 300g granulated sugar, and lightly crush the blueberries with a rolling pin to mix them well with the sugar. Let the blueberries marinate in the sugar for 1 hour to release their juice and enhance the sweetness.

Pour the marinated blueberries and their juice into a non-stick pan. Bring to a boil over high heat. As foam forms on the surface, skim it off with a spoon to ensure the jam is pure and smooth.

After removing the foam, reduce to medium-low heat and simmer. Stir constantly to prevent sticking or burning. Cook until the blueberries are soft and the mixture thickens. This will take around 20–30 minutes, but the time may vary depending on your desired consistency.

When the jam reaches a soft and thick texture, juice 2 fresh lemons and add the juice to the pan.

Stir well and continue simmering until the lemon juice is fully incorporated, and the jam thickens further. To test the consistency, scoop up some jam with a spoon. If it drips slowly and forms a slight mound on the spoon, it's ready.

While the jam is still hot, transfer it into sterilized jars. To sterilize, wash the jars, submerge them in boiling water for 10–15 minutes, and let them dry completely.

Fill the jars as much as possible to minimize air inside, then seal them tightly with lids.