To make the rhubarb syrup, place the rhubarb, sugar, and orange zest and juice in a saucepan and add a splash of water. Bring to a boil, then reduce the heat to low and simmer gently, uncovered, for 10–15 minutes until the rhubarb is very soft.
Strain the syrup into a pitcher, pressing the rhubarb to extract all the juice, then let cool.
Pour a little of the rhubarb syrup into each glass and top off with chilled sparkling wine.