Test the walnuts: Pierce one walnut with a toothpick or skewer. It should pass through easily, without meeting a hard or woody shell. Use only young, tender walnuts.
Prepare the lemon water: Put on protective gloves. Fill a large bowl with cold water and add the juice from 2 lemons.
Peel the walnuts: Using a small sharp knife, remove the dark green outer skin from each walnut. Leave a little of the pale green layer underneath for a deeper color and stronger walnut flavor. Place every peeled walnut immediately into the lemon water to limit oxidation.
Pierce the walnuts: Push a skewer all the way through each walnut, making small holes on opposite sides. Repeat 3–4 times per walnut, or as much as the walnut allows without breaking.
Soak the walnuts: Leave the pierced walnuts in the first water for about 15 minutes. Drain, cover them with fresh cold water, and add the juice from 2 more lemons. Weight the walnuts down with a plate and refrigerate for approximately 16 hours.
Rinse the walnuts: The following day, drain the soaking water and rinse the walnuts thoroughly.
Blanch the walnuts: Place the walnuts in a pot and cover them with fresh water. Bring to a boil, which may take approximately 15–20 minutes, then boil for 5 minutes. Drain and transfer the walnuts to cold water.
Drain and weigh: Drain the walnuts thoroughly and weigh them. The original 2.5 kg of whole walnuts should produce approximately 1.5 kg of prepared walnuts.
Make the syrup: Combine the sugar and 300 ml water in a wide, high-sided pot. Bring to a boil, then cook for 6–7 minutes, until the syrup thickens slightly. This stage takes approximately 20 minutes in total.
Test the syrup: Place a drop of syrup on a room-temperature plate. It should hold its shape like a small dome instead of running immediately like water.
Cook the preserve: Add the drained walnuts and the remaining lemon, thinly sliced. Allow the mixture to return to a boil, then simmer for 10 minutes. This stage takes approximately 25–30 minutes in total.
Rest overnight: Remove the pot from the heat and cover it with a clean damp towel. Leave the preserve to cool and rest overnight. The damp towel helps prevent sugar crystals from forming on the surface.
Check the consistency: The following day, check the cooled syrup. If it is too thin, return the preserve to the heat, bring it back to a boil, and cook for another 5 minutes. It can be boiled for up to 10 minutes if necessary, but remember that the syrup will continue to thicken as it cools.
Cool completely: Remove the pot from the heat, cover it again with the damp towel, and allow the preserve to cool completely.
Fill the jars: Transfer the completely cooled preserve into sterilized, slightly warm jars. Distribute the walnuts and syrup evenly, seal the jars, and store them in a cool, dark pantry.
