Put the rhubarb in a pan with the sugar & heat gently, stirring occasionally until the sugar is dissolved. Increase the heat to a simmer & cook for 5-6 minutes, until the rhubarb is tender. Remove from the heat & allow to cool.
Put the mascarpone in a bowl & beat in the custard, orange rind & juice until smooth. Fold in the rhubarb & meringue nests until just combined.
Spoon the mixture into a rgid freezer container & cover. Freeze for 1-2 hours, until the mixture starts to harden around the edges & then beat well with a fork to break up the ice crystals.
Returen to the freezer & repeat this process once more. Finally, freeze for 2-3 hours until firm. Remove the ice cream from the freezer around 20 mins before serving.