Add the gin, lemon juice, lemongrass syrup and 6 smacked/expressed mint leaves into a shaker with ice and shake until well-chilled.
Double-strain into a coupe glass.
Top with brut cava.
Garnish with a mint leaf that has been smacked between your hands to express the oil.
*Lemongrass syrup: Combine 1 cup sugar, 2 peeled lemongrass stalks (sliced) and 1 cup water in a small saucepan. Bring to a simmer and continue to simmer for 10 minutes or until desired flavor is achieved. Remove from heat, cool and strain out solids. Store in the refrigerator for up to one week.