Remove the peel and seeds from the watermelon, cut into pieces and puree in a stand mixer with a little cinnamon.
Measure 500 ml of watermelon puree, stir about 100 ml of it with the custard powder. Bring the rest to a boil with sugar, stir in the custard powder mixture with a whisking rod and bring to a boil a few times.
Divide into 4 small bowls, cool and let set in the refrigerator.
Mix the rest of the watermelon puree with the lemon juice and pour over the watermelon pudding.