Rim two cocktail glasses with sugar. Set aside.
To make the cocktail, add mezcal, lemon juice, clementine juice, and simple syrup to a shaker filled with ice. Shake well.
Fill cocktail glasses with ice and strain cocktail into both glasses, dividing evenly.
Top with club soda and give a final stir.
To make the brûléed clementine slices, top each with 1 1/2 tsp sugar. Using a kitchen torch, slowly go back and forth over the slices to melt and brown the sugar. Let set for about 1 minute to let the melted sugar harden.
Garnish cocktails with a brûléed clementine slice each and rosemary sprigs. Serve immediately.