Combine sugar, water, prunes, cherries and 6-8 rosemary leaves in a saucepan. Boil for 15 minutes, remove from heat and mash fruit. Let stand an hour then strain into a container and refrigerate.
When ready to serve, fill the glass with ice and arrange a couple of slices of fruit in the glass. Pour in 3 oz of plum simple syrup and top with Prosecco. Add a sprig of rosemary. Serve and enjoy!
*If plums are in season use 2 plums in place of prunes.