Cut watermelon flesh into ice cube-size pieces and freeze on a sheet tray lined with wax paper until solid, about 2 hours.
Shake sherry with lemon juice, simple syrup and ice in a cocktail shaker. Place 3 to 5 watermelon cubes in a Collins glass (save remaining watermelon cubes for another use). Strain the liquid over the cubes. Top with cava.
This recipe has been updated and may differ from what was originally published or broadcast.