Combine the rhubarb, sugar and 2 cups medium water in a medium saucepan.

Bring to boil over medium-high heat and cook, stirring occasionally, until the rhubard is slightly broken down. (about 10 minutes)

Remove from heat and add the mint; let sit 1 minute, then strain.

The mixture should be syrupy; if it's too watery, return to the saucepan and continue cooking until slightly reduced. Cover and refrigerate until cool.

Divide the rhubarb syrup among 6 champagne glasses and top with the Prosecco.