To make the rosemary simple syrup, combine the water, honey, and rosemary in a small saucepan. Over medium heat, stir until the honey is completely dissolved, about 1 minute. Remove from heat and let cool for 30 minutes. Remove and discard the rosemary sprig.
In a glass, combine the cider, seltzer water, and 2 tbsp of the simple syrup and garnish with rosemary sprig.
