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 the boil, then reduce the heat to low and simmer gently, uncovered, for 10–15 minutes until the rhubarb is very soft.
Strain the syrup through a fine sieve into a jug, pressing the rhubarb to extractall the juice, then leave to cool.
Pour a little of the rhubarb syrup into each glass and top up with chilledsparkling wine.