Combine first 3 ingredients in 9oz old-fashioned glass over ice and stir until syrup is integrated. Top with soda and stir. Garnish with rosemary sprig.
An aggressive, smokey scotch works best, but trying different styles of scotch could be fun! No need to use expensive calvados (or scotch, for that matter). Make sure to smack the rosemary around a bit before garnishing to release more fragrance.