Muddle four blackberries, 1 sprig of thyme, and the juice of ½ a lemon in a cocktail shaker until very smooth.
Fill shaker halfway with ice and add gin and St. Germain. Shake until well chilled.
Strain into a double old fashioned glass filled with ice. Top with Fever Tree Elderflower tonic water and stir with a cocktail straw to combine. Garnish with a lemon slice, fresh blackberry, and a sprig of fresh thyme.
Recipe by The Taste Edit at https://thetasteedit.com/blackberry-elderflower-gin-and-tonic/