Mix the watermelon flesh with the sugar and lemon juice and leave to stand for about 1 hour.
Soak the gelatine in cold water.
Push the watermelon through a sieve, reserve 1 tbsp of the melon puree and make the rest up to 1/2 l/18 fl oz with apple juice.
Heat a little of the liquid, squeeze out the gelatine and stir into the heated liquid to dissolve.
Add the rest of the watermelon liquid, stand the container in a bowl of cold water and stir until the mixture begins to set.
Transfer to 4 individual moulds.
Whisk the egg white very stiffly with the sugar and mix in the reserved melon puree.
Then pipe a blob on the top of each jelly.
Stand the moulds in a baking dish filled with cold water and put in the oven at 200 degrees for 5 minutes.
Then put into the fridge for about 3 hours to set. Served dusted with icing sugar.