Combine the rhubarb, sugar and 2 cups water in a medium saucepan. Bring to a boil over medium-high heat and cook, stirring occasionally, until the rhubarb is slightly broken down, about 10 minutes.
Remove from the 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.
Photograph by Kang Kim
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