Step 1

Chill 2 Collins glasses. Combine rosemary leaves and salt in a cocktail shaker and gently press out oils with a muddler, about 45 seconds. Add egg white, lemon zest, mezcal, lemon juice, and agave nectar. Cover cocktail shaker; shake vigorously (without ice) until egg white is emulsified, about 20 seconds. Carefully uncover (shaker will be quite full) and add ice just to fill. Cover and shake until egg white is very frothy and cocktail is cold, about 30 seconds.

Step 2

Set chilled glasses next to each other and double-strain cocktail into glasses, dividing evenly. Top off each drink with 3 oz. club soda and garnish with a rosemary sprig.