In a medium saucepan over medium heat, combine the berries, water, and sweeteners. Bring to a simmer and cook until the berries can be mashed easily, about 5 minutes.
Remove from heat and strain the syrup through a fine mesh sieve to remove the seeds. Press on the solids to release as much juice as possible.
Fill a high ball glass with ice, then add club soda to about 2 inches from the top. Add ¼ cup of the raspberry syrup, then stir in one tablespoon of heavy cream.
Top with lightly sweetened whipped cream, if desired.
*Nutritional information assumes that some of the carbs and most of the fiber was lost in straining the syrup through a sieve. The majority of the fiber from raspberries is in the seeds and those are discarded.