Add a few ice cubes to a glass. Then add the Rhubarb Syrup and elderflower liqueur. Top off with prosecco, stir and serve.
For the rhubarb syrup: In a medium saucepot, add the rhubarb, sugar and 1/2 cup water. Bring to a gentle simmer over medium-low heat and cook until the rhubarb is tender and the liquid turns bright pink, 15 to 20 minutes. Strain the syrup and let cool completely.