Place bean thread noodle in boiling water until al dente, about 2 minutes. Strain and chill until cool to the touch.
In a medium bowl, combine the sesame oil, sesame seeds, rice vinegar, mirin, brown sugar, ginger root juice, salt and pepper to taste and chili pepper (if using). Whisk to combine until smooth, then slowly pour in the canola oil while continuing to whisk.
Add the crab, cucumber, taegu, ¼ cup green onion, bean sprouts and watercress; toss lightly to combine. Add in the noodles.
Just before serving, toss the noodles in the dressing. Garnish with black sesame seeds and sliced green onion if desired and enjoy with a nice cold lager beer.