This is the soup I mentioned in my post on Delia. I did not use the "English Provender very lazy ginger" she recommends; the pinch of cayenne is my addition. Her soup includes a coriander garnish; but are you going to buy a bunch of coriander specially?
Delia recommends the Blue Dragon brand, which I like too. I suspect that other brands of coconut milk would also be fine. Normally, I steer clear of "light" products; but full-fat coconut milk is very fatty.
You may find that your soup contains tough little strands of lemongrass, which is hard to mince finely.
415g tin lobster bisque ("such as Baxters", Delia says)
165ml tin light coconut milk
1 stalk Blue Dragon lemongrass (from a jar)
Fingernail-sized piece ginger
Pinch cayenne pepper (or more, to taste)
1dstsp fish sauce
1tbsp lime juice
Mince the lemongrass and ginger, in a small electric mill if you have one. Warm the bisque, coconut milk, lemongrass, ginger and cayenne in a small saucepan over a gentle heat; when it is steaming, turn the heat down low, and warm it for a further 10 minutes, stirring regularly and without letting it boil. Add the fish sauce and lime juice, and serve.
Warning: this soup has an assertive flavour, which will linger.