To make the broth, mix together the stock, palm sugar and fish sauce in a pot and bring to the boil. Remove the broth from the heat, add the sesame oil and keep warm.
Dissolve the palm sugar in the fish sauce, then whisk in the eggs. Gently mix together the bean sprouts, snow pea sprouts, chives and crabmeat in a separate bowl.
Heat a wok until just smoking. Add the oil and, when hot, pour in the egg mixture and cook for 3 minutes, moving the egg around the wok until the bottom is golden brown. Spoon the crab mixture into the centre and turn down the heat. Pour off any excess oil and flip the omelette over so that the crab is underneath the egg. Leave for 1 minute, then transfer the omelette to a chopping board, trim the ends and place in a large bowl. Pour in the hot broth and drizzle with oyster sauce to serve.