Cut the rhubarb into 1″ lengths. Slit the cardamon pods and extract the black seeds then roughly grind the seeds in a pestle and mortar.
Put the rubarb in a pan with the sugar and cardamon seeds. Cook, covered,over a low heat for a few minutes until the juices begin to run. Raise the heat and simmer for 2-3 minutes until the rhubarb is tender.
Scoop out the rhubarb and set aside, then boil the juice until it has reduced in quantity by half. Turn off the heat and leave to cool.
Pour boiling water over the pistachio nuts and leave for 30 seconds before draining and rubbing off the skins. Spead on a baking tray and dry in oven at 140 c Gas 1 for 5-10 minutes. Roughly chop.
Whip the cream until it holds its shape. Fold in the cooked rhubarb and juices and 2/3 of the chopped nuts.
Divide between 6 individual bowls and then scatter with the remaining nuts. Chill until ready to serve.