Combine the rhubarb, sugar, water, lemon zest and juice in a small saucepan and bring to a simmer. Lower the heat to a slow bubble and cook until the rhubarb softens to a mushy texture, about 15 minutes.
Puree the rhubarb with an immersion blender or small food processor until smooth. Refrigerate until cold. Or if you're very thirsty, chill the puree in a bowl of ice water until cold.
Pop open a bottle of cold prosecco or other sparkling wine. Spoon 1-2 tablespoons of the puree into the flutes. Pour some prosecco over, stir to blend with a long spoon or a chopstick and top off with more Prosecco, pouring gradually (the mixture may bubble madly for a minute).