To cook the geoduck, remove the siphon (the neck part) from the shell. Bring a medium pot of salted water to a boil, add the geoduck and cook for about 20 seconds. Immediately immerse in iced water. Drain and remove the outer skin and discard. Cut in half lengthwise and thinly slice on a bias (the thinner the better as it will marinate and pick up more flavor). In a stainless steel bowl mix together the prepared geoduck, onions, and mushrooms. Cover and set aside in refrigerator for about 20 minutes.
In a small bowl add the oil, rice wine vinegar and mustard and combine well. Adjust the seasoning with salt and pepper, to taste. At this point carefully add hot sauce to your liking. (Remember the more you add the hotter it gets.) To finish, add a little dressing to the geoduck salad and let marinate for a further 20 minutes in the refrigerator.
Toss the chopped romaine lettuce, chives, cilantro and mango in a serving bowl and top with the geoduck salad.