Add the chopped cucumber and grated ginger to a bowl. Mash thoroughly with a muddler or wooden spoon until the cucumber releases a lot of juice and the mixture becomes intensely aromatic. Let stand for 5–7 minutes. This pause matters — it builds depth and gentle heat. Skip it and the drink tastes hollow.
Strain the mixture through a fine-mesh sieve into a pitcher. Press lightly to extract juice, but do not force pulp through. Discard the solids. The liquid should be clean, pale green, and fresh — not cloudy sludge.
Add 80 ml elderflower syrup, the fresh lime juice, 1 tablespoon honey, and the pinch of fine sea salt to the pitcher. Stir gently and taste adjust carefully, this base should taste slightly too intense — soda will dilute it.
Add plenty of ice to the pitcher. Pour in the chilled club soda last and stir gently once or twice. No more. Carbonation is fragile. Abuse it and the drink goes flat.
Pour into ice-filled glasses and garnish with long cucumber ribbons and optional edible elderflower blossoms.