Add 1/2 fluid ounce Tanqueray No. Ten gin (750ml, $44.99), 1 fluid ounce grapefruit juice, 3/4 fluid ounce rosemary syrup and 1/2 fluid ounce lime juice to an ice-filled highball glass leaving enough room for a few splashes of soda. Top with a dash of soda, give the glass a quick stir and garnish with a sprig of rosemary.
To make a rosemary syrup, combine 1 cup of simple syrup with the leaves from one sprig of rosemary in a blender; purée on high for 1 minute. Pour the syrup through a fine strainer into a clean jar. The syrup will keep for up to 1 month in the refrigerator.