To make the sauce, slice the vanilla pods along the length, and halve the lemongrass lengthways. Boil the fish stock, vanilla pods and lemongrass in a small pan, then turn off the heat, cover and allow to steep for approx. 15 mins. Drain, remove the lemongrass, and place the vanilla pods on one side. Pour the liquid back into the pan and reduce to approx. 200 ml.
Slice the carrot and cabbage into thin strips, add to the sauce, simmer for a further 5 mins. or so with the lid off. Scrape out the vanilla seeds, add with the double cream, reduce the sauce a little and season.
Heat the clarified butter in a frying pan and fry the scallops for approx. 1 min. on each side. Plate up and serve with the sauce.