Cut the spring onions into thin strips and put them in cold water.
Lightly beat the egg whites. Mix the white and black sesame seeds with salt and pepper on a large plate. Dip the salmon fillet in the egg whites then roll it in the sesame seeds. Pat the salmon on the seeds all over to give a good, even coating. Heat a griddle pan, add the salmon and cook for 2 minutes each side for rare or 5 minutes for well done.
Make the dressing by mixing together the vinegar, oils, soy sauce, caster sugar and chives. Toss the frisée leaves and watercress in the dressing. Arrange the leaves on a large serving dish.
Finely slice the salmon fillet and place on top of the salad. Drain the spring onion curls, dry them on kitchen paper and sprinkle over the salmon. Serve immediately.