Bring a large pot of water to a boil. When it is boiling, plunge the peaches in the boiling water and blanch them for 2 minutes. Remove the peaches from the boiling water and drop them into a bowl of ice-cold water. Now you should be able to peel them effortlessly.

Once peeled, cut the peaches into wedges, discard the stones, and collect them in a large, shallow pot.

Squeeze the lemon over the peaches, then add also the squeezed lemon into the pot. It will release pectin, which will help to have a thick jam.

Cook the peaches over low heat for about 20 minutes, or until they begin to collapse. Stir often.

When the peaches are soft, remove the two half lemons, then purée the peaches with an immersion blender.

Add the sugar and stir well until melted.

Put the pot back on the stove and bring to a boil, then simmer for about 20 minutes. If you have a pastry thermometer, the jam will be ready when it reaches 105°C (221°F). The jam will still be very liquid, it will thicken up once cooled down.

You can also check empirically whether the jam is ready or not with the saucer test. Pour a drop of jam onto a cold saucer from the freezer. If it thickens and does not slip away when you tilt the saucer, the jam is ready to be poured into sterilized jars. Close the lids tightly.

Now boil the jars placed in a large pot and covered with water for about 20 minutes. Then turn off the heat and let the jars cool down completely before removing them.

They can be kept in the pantry for more than a year.