Make simple syrup: In a small saucepan, combine the sugar, honey and water. Heat over medium high heat, stirring occasionally until the sugar is dissolved.
Tie the thyme sprigs together with a piece of kitchen string. Submerge the thyme in the still hot simple syrup and steep for 30 minutes.
Remove the thyme and discard. Strain the syrup through a fine mesh sieve to remove any thyme leaves or other particles. Store in a glass jar in the refrigerator.
For each cocktail
Into a champagne flute combine simple syrup, grapefruit juice and chilled prosecco. Garnish with a wedge of grapefruit and fresh thyme. Serve.
I used 1 tablespoon of the simple syrup, but if you prefer a sweeter cocktail, add two tablespoons.