Boil the 375g rhubarb with the sugar and water for 5 minutes, skim and pass.
Discard the pulp. Add the scraped vanilla pod to the syrup and bring back to the boil. Add the trimmed lengths and poach for 1 minute – leave to cool, then chill.
For the mousse
Heat the tangerine juice with half the zest until it reduces to 30-40ml. In another pan bring the water and sugar to the boil to dissolve and add the juice mixture. It should have a consistency of marmalade.
Bring the egg yolks to a light and fluffy state over hot water, continue to beat and add (slowly) the juice mixture. Remove from the heat and continue beating till cold with a hand whisk. Whip the cream to soft peaks and fold into the egg mixture. Fold the other half of the zest into the mixture. Pour into 6 ramekins or suchlike and freeze for 6 hours.
To serve, divide the sauce between 6 soup plates and dip each of the moulds in very hot water and set in the middle of each. Dust with icing sugar and serve.