Combine all ingredients and 1 cup water in saucepan and bring to a gentle simmer. Simmer, stirring occasionally, until sugar has dissolved. Remove from heat; cool completely. Strain through fine sieve, using the back of a spoon to press and release juice from grapefruit. Fill a 5 oz. coupe glass 3/4 full with Cinzano Prosecco. Add the Cinzano Extra Dry Vermouth, simple syrup, and a splash of grapefruit juice and stir to combine. Garnish with grapefruit slice and fresh thyme.
