1. Combine the rhubarb, sugar and two cups water in a medium saucepan. Bring to a boil over a medium-high heat and cook, stirring occasionally, until the rhubarb is slightly broken down, about 10 minutes.
2. Remove from heat and add the mint; let sit for one minute, then strain. The mixture should be syrupy; if it is too watery, return to saucepan and continue to cook until slightly reduced. Cover and refrigerate until cool.
3. Divide the rhubarb syrup into 6 champagne glasses and top with prosecco.