Peel and slice half of the watermelon into chunks and put in a freezer-proof container.

Pour the vodka over the watermelon making sure that most of it soaks into the fruit.  (You can put the lid on the container and shake it a bit to help with this!)

Freeze the vodka-soaked watermelon for a few hours.

Squeeze half the lime and peel and grate the ginger.

Add the frozen watermelon to your blender or food processor, along with the lime juice, mint and fresh ginger. 

Blend everything for a few seconds until the watermelon has become liquid

Slice the other half of the lime into rings and use to garnish your glass.

Serve quickly so the drink remains slightly frozen.