Add the cranberries to food processor and purée into a smooth paste.
Add the cranberry purée to a small saucepan with the simple honey syrup and rosemary. Simmer over medium heat for 5 minutes, until slightly reduced. Let cool completely, then strain into an airtight glass container and store in the refrigerator for up to 2 months.
To serve, mix 1-2 tablespoons of cranberry rosemary syrup with 8 ounces of seltzer water in a glass over ice.
Nutrition, per 1 tablespoon
Sugars: 4 grams