Put the pot to boil with the water, salt and sliced ginger, covered with a lid. Arrange beef slices at the side of 4 individual plates. Pat scallops dry, discard crescent-shaped membrane and add to the plates. Cut bean curd in large cubes and add also to the plates. Discard stems from spinach and pile on plates. If using watercress, rinse the bunches, shake dry and twist off the stems. Pile leaves on the plates. Trim bok choy, cut in 1 inch slices and pile also on the plates. Trim and add the scallions. Chop the coriander and put in a small bowl to be added to the firepot with the rice noodles.
For the sauce: stir together the sesame paste, soy sauce, chili paste and vinegar and adjust proportions to your taste. Put in 4 individual bowls. Transfer the pot to the table burner. Set plates of meat and vegetables at each place, with chopsticks and a bowl of sauce.
Towards the end of the meal, add rice noodles and coriander to the pot and simmer 1 minute until just tender. If your firepot loses too much heat during the meal, add all the remaining ingredients and return the pot to the stove to finish cooking.