1. Prepare the sauce: Add enough cold water to the mustard powder to make a thin paste; let stand 10 minutes. In a food processor combine mustard paste, sesame paste, sesame oil, vinegar, soya sauce, garlic, sugar, fish sauce (if using), pepper and salt; purée until smooth. Add enough cold water to make a thick but pourable sauce. (The sauce can be refrigerated for 5 days; bring to room temperature before using).
2. Bring 3 cups (750 ml) water and salt to a boil. Blanch celery 30 seconds; remove, rinse in cold water, cut diagonally into bite-size pieces and set aside. Add peppercorns, ginger, whole green onion and coriander, if using, to the boiling water. Add chicken breast, skin-side up; when water returns to a boil, reduce heat to low, cover and cook 12 to 15 minutes or until chicken is cooked through but still moist. Remove from water; cool. Strain liquid and save for use as a light stock. Remove and discard skin and bones; shred chicken meat or slice thinly.
3. Line serving platter with Napa cabbage leaves; cover with radish and cucumber, then celery and chicken. Drizzle sauce in lines over the salad. Sprinkle with chili, red pepper, coriander and, if desired, sesame seeds.
Some northern Chinese and, especially, Koreans prefer a sauce with a pronounced mustard taste and sinus-clearing piquancy. For this, make sure you use hot English mustard powder. Or, for the best and hottest mustard powder, buy the Korean product at Korean grocers. Avoid Chinese mustard powder, as it is inferior to the English variety. Increase the mustard to 1 tbsp (15 ml).
Wasabi, Japanese green horseradish, can also replace the hot mustard. Use 2 tsp (10 ml) powdered wasabi and add enough cold water to make a thick paste; let it sit covered 10 minutes before adding to the sauce.
If you ever see fresh radish sprouts at your grocers, buy them for this salad. The attractive sprouts are absolutely delicious and have the tangy flavor of radish. Onion sprouts can replace the green onion in the salad. Chopped pickled young ginger also makes for a very nice garnish.