Cut the watermelon into chunks.
Puree it in a blender or food processor. Put two cups of puree in a pot. Set 1/4 cup of puree aside in a small bowl. Put the rest of watermelon puree in the pot on a medium heat and bring it to a boil
. Add the gelatine to the 1/4 cup of watermelon puree in the bowl and mix it in well.
When the watermelon puree reaches a boil, take it off the heat and stir in the gelatine watermelon mixture.Blend it together well, so you don't have any gelatine lumps. Add in 1/2 Tbs of lemon juice
Add in 1/2 Tbs of lemon juice, and 1/2 cup of sugar.Mix everything together well and set it aside to cool.When it has cooled, whip the cream.
Fold the whipped cream into the cooled watermelon mixture.
Ladle the watermelon mousse into small serving ramekins. Put the ramekins into the refrigerator to chill until they set, usually two hours or so.
Decorate each one with a dab of whipped unsweetened cream and serve them up.