Muddle the tarragon in a cocktail shaker. Fill the shaker with ice, grapefruit juice, gin, St. Germain and the simple syrup. Shake and strain into a tall glass (Collins glass) filled with ice. Top with club soda, a grapefruit slice and tarragon, if desired.

To make the simple syrup combine 1 cup sugar and 1 cup water in medium saucepan. Bring to a boil. Stir until all the sugar is dissolved. Let cool.