juice of one whole fresh lemon, separated into halves (to be used separately)
Put two glasses in the freezer to chill.
Make the quick blueberry sauce: heat the blueberries, sugar, water and juice of half lemon in a small saucepan over low heat for about 10 minutes, stirring occasionally until the blueberries break down a bit and the sauce thickens. Strain through a mesh sieve and set aside to cool.
Blend the watermelon, ice, the rest of the fresh lemon juice, gin, and basil on high until just combined. It should be thick and slushy – if you want it even slushier, add more ice.
Take out each chilled glass and spoon half the blueberry puree jam into the bottom, rubbing it around the bottom sides as you go. Pour in the watermelon slush. Serve with more blueberries and basil and a straw for slurping.