Cut the rhubarb into 1-inch lengths.
Slit the cardamom pods and extract the black seeds.
Crush.
Put the rhubarb, cardamom seeds, and sugar in a pan and cook, covered, over a fairly low heat for a few minutes until the juices begin to run.
Raise heat and simmer for 2 to 3 minutes until the rhubarb is tender.
Scoop out the rhubarb and boil the juice hard until reduced by 1/2.
Turn off the heat and cool.
Pour boiling water over the pistachios and leave for 30 seconds.
Drain and rub off skins.
Spread out on a baking tray and dry in an oven at 325° F. for 5 to 10 minutes.
Chop.
Whip the cream and until it holds its shape.
Fold in the cooked rhubarb and juices and 2/3 of the chopped pistachios.
Spoon into individual bowls and scatter with the remaining nuts.