Method
Place the diced watermelon pieces into a blender. Pulse until you have a completely smooth puree.
Pour in the fresh lemon juice, sugar, and the chilled sparkling mineral water. Give it a quick pulse or a gentle stir to combine.
Pour the mixture through a fine-mesh strainer into a pitcher or a large glass. This removes any remaining pulp or fibers for a crystal-clear finish.
Fill your serving glasses to the brim with ice cubes.
Pour the sparkling watermelon mixture over the ice. Garnish with a sprig of fresh mint or a small wedge of watermelon on the rim.
Serve immediately while the fizz is at its peak!
Notes
To keep the drink fizzy, gently stir after adding the sparkling water and serve immediately — overmixing will reduce carbonation.