Preparation

• Pour the gin, ¾ ounces rhubarb rosemary syrup, lime juice, and egg white (if using) into a cocktail shaker. Cover and vigorously “dry shake” for about 30 seconds to combine.
• Add enough medium ice cubes to nearly fill the shaker, cover and shake as vigorously as you can until frothy, at least 30 seconds. Strain into an ice-filled tall Collins or chimney glass.
• Drizzle or drop the remaining rhubarb rosemary syrup over the top. Garnish with the rosemary sprig.

Rhubarb Rosemary Syrup:

• Stir together the sugar and 2 cups water in a medium saucepan over medium-high heat. Bring to a boil, stirring occasionally until the sugar is dissolved.
• Add the sliced rhubarb and reduce the heat to low. Continue cooking until a syrupy consistency is reached.
• Remove from the heat and add the rosemary sprig; let stand 30 minutes.
• Strain through a wire-mesh sieve lined with a double layer of damp cheesecloth, discarding the solids. Store refrigerated in an airtight container up to 1 month.