Add the chopped rhubarb, sugar and water to a large, heavy-bottomed saucepan and bring to a boil over medium high heat, stirring to dissolve the sugar. Reduce to a simmer, and cook for 10 minutes, or until the rhubarb is very soft and jam-like. Remove from heat and allow to cool to room temperature. Strain through a fine mesh sieve, pressing down on the rhubarb to extract as much liquid as possible. Discard the solids or save for another use. Make ahead: Store the rhubarb syrup in a jar with a tight fitting lid for up to 5 days.
Rhubarb Gin Spritz
Fill a large glass with ice. Add 1 oz rhubarb syrup, 1 oz gin and 1/2 oz lemon juice. Top with 3-4 oz of prosecco and stir gently. Garnish with candied rhubarb ribbons, if using. Serve immediately.