Add all the ingredients to a shaker with ice, and shake until well-chilled.
Strain into a chilled coupe glass, and garnish with a rosemary sprig.
*Salted cucumber juice: Using a juicer, juice peeled cucumbers to yield 1 quart. Strain juice to remove any pulp, and add 1 to 1 1/2 tsp sea salt to add flavor and preserve the juice. Store the mixture in the refrigerator.