Fill shaker with whiskey, rosemary simple syrup*, and ice. Shake for 30 seconds. Pour whiskey mixture evenly into eight whiskey glasses.
Place a large ice block into each class. Top each cocktail with a splash of sparkling apple cider. Garnish with rosemary sprig and apple slices.
*Note: To make rosemary simple syrup, bring one cup of water and one cup of sugar to a simmer. Remove from heat and add in three 5-inch rosemary sprigs. Steep for 20 minutes and strain mixture into a jar.