Cut the mandarin pulp, including the white skin, into small pieces and mix with the juice of the lemon and the preserving sugar. Let something stand.
Then puree the mixture and let it boil for 15-20 minutes, skimming off the foam that forms. Pour hot into clean lid jars and screw them upside down for 10 minutes.
The fact that the white skin stays on the fruit means that less sugar is needed because the skin contains pectin.
If you like, you can refine the jam with the pulp of a vanilla pod.
The amount is sufficient for 2 glasses of 220 ml each.