Lemonade base: In a blender or food processor, puree the blueberries with sugar, 50 ml of still water and lemon juice until smooth. Strain the puree through a fine mesh sieve to remove any seeds, if desired. You can keep the blueberry pulp to stir into a yoghurt or smoothie
Assemble: add the lemonade base to a large jar that can hold up to 1L. Add the lemonade base and fill up with sparkling water.
Serve: Serve with ice cubes and some fresh blueberries and lemon slices