1 cucumber, a few long slices reserved for garnish (optional)
400ml Fever-Tree Elderflower Tonic
60ml simple syrup
120ml Hendrick's Gin
½ cup sugar
½ cup water
For the simple syrup: Heat the sugar and water in a small pot until sugar is completely dissolved. Leave to cool. Extra sugar syrup can be kept for up to 1 week.
For the cucumber and elderflower granita: Puree the cucumber and sugar syrup in a blender, then fine strain liquid into a freezeable, non-porous bowl, discarding solids. Add tonic water and gin and stir well. Place in freezer and periodically scrape with a fork as it starts to freeze. When fully frozen scrape into icy piles.
For the cocktail: Put a couple of spoonfuls of granita into champagne glasses Pour sparkling wine to top. Slice long thin strips of cucumber and concertina onto cocktail skewers to garnish, if desired.