40 rosemary leaves (from 1 sprig)
Pinch of kosher salt
1 large egg white*
1 3"-long strip lemon zest
4½ oz. mezcal
2¾ oz. fresh lemon juice
1½ oz. light agave nectar
6 oz. chilled club soda
2 sprigs rosemary, briefly singed over flame of a gas burner if desired
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.
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.