Brown the sesame seeds in a dry pan. Chop off the ends of the green asparagus and discard them. Peel the top bit of the asparagus stalks. Cut off the tips of the asparagus and chop the remaining parts into pieces. Blanch the tips briefly. Stir-fry the pieces of asparagus (except the tips) and add the lime leaves, shallot, ginger, lemongrass, red chilli and garlic. Fry briefly with the pieces of asparagus and then add the coriander and cumin. Add the stock and coconut milk and cook until the asparagus are tender.
Remove the lime leaves. Puree the soup. Slice the asparagus tips in two, slice the spring onion into rings and fill the soup bowls with them. Ladle the soup on top, garnish with the deep-fried onions, sesame seeds and coriander.