A salty splash from preserved lemon gives this cocktail a new lease on life.
Using a sharp knife, de-pulp the lemon. Ditch the pulp, but save the peel. Give the peel a light rinse with cool water, then slice off a little sliver for the cocktail garnish.
Combine the rest of the lemon peel with the gin in a shaker. Muddle the lemon until it's slightly crushed. Add 1 cup of ice to the shaker, put the cover on, and shake it up until it's chilled, 15 to 20 seconds.
Fill a rocks glass with ice, then strain the shaker contents over it. Top it off with tonic water, and garnish with the lemon sliver and rosemary sprig.