Place the clementines in a bowl of water and leave to soak overnight.
Pierce the fruit from top to bottom and on all sides with a skewer.
Place the clementines in a large saucepan, sprinkle with sugar and lemon juice, and pour in the water. Leave the sugar to dissolve over low heat.
Cook for a total of 2 hours, in three or four stages, allowing the fruit to cool between each stage.