Remove and set aside the red flesh from the watermelon, leaving about one centimeter of rind.
Remove the dark green outer rind with a sharp knife or peeler.
Cut the light green rind and the red pulp into bite-size pieces.
Add two liters of water and 200 ml of apple cider vinegar. Bring to a boil and let everything cook on medium heat for about 2 hours.
Pour the rind mixture into a glass bowl and chill with cold water.
When cooled, put the rind mixture back into the pot. Add half a liter of water, the juice of one lemon, a kile of jam sugar and a packet of vanilla sugar. Cook everything until the mixture has thicked.
Puree the mixture partially or completely according to your desire.
Pour the hot jam into sterilized jars. Close the jam jars and turn them upside down for 5 minutes. Leave the jam jars in a kitchen towel for half a day.
