Bring the cream, milk, salt and cardamom to the boil in a pan. Using the whisk on a hand mixer, beat the egg yolks and sugar in a bowl for approx. 3 mins., continue to beat until the mixture thickens and becomes lighter in colour. Add the hot cream mixture while stirring, pour through a sieve into a measuring cup, pour into the ramekins. Place the ramekins on a cloth in an ovenproof dish. Pour enough boiling water into the dish to come two-thirds of the way up the side of the ramekins. Cover the ramekins individually with foil.
To cook in the oven
Approx. 30 mins. in the centre of an oven preheated to 160°C. Remove, leave to cool, cover and chill for approx. 3 hrs.
Mix the rhubarb, water and sugar in a pan, simmer for approx. 5 mins. until just soft, leave to cool. Mix in the lime zest and lime juice, spread on top of the custard pots, sprinkle with the pistachios.