To make the broth, combine the stock, ginger, garlic, scallions, soy sauce, and sesame oil in a large saucepan and bring to a boil over high heat. Reduce the heat to medium and simmer, uncovered, for 20 minutes. Strain, discard the solids, and return the stock to the saucepan. Keeping the heat at medium to medium-low, maintain a slow, steady simmer while preparing the dish. Add the cabbage and simmer for 1 minute. Use a slotted spoon to transfer to a large platter. Do the same for the carrots, peppers, and mushrooms, cooking the carrots and peppers for 3 minutes and the mushrooms for 2 minutes. Arrange each in a separate mound on the platter as they are cooked.
Place the beef in the freezer for about 30 minutes or until slightly stiff. This makes it easier to slice thinly. Cut the beef against the grain into paper-thin slices and arrange decoratively on a different platter. Add the peas to the platter.
To make the sauce, combine the sour cream, wasabi, chives, and mustard in a small bowl. Thin with water as desired. Stir well and season with salt and pepper.
Set the fondue pot in the middle of the table, fill it with the broth, and bring to a simmer. It is up to each of your guests, in turn, to finish cooking this dish. Diners choose the vegetables they want and place them in their empty soup bowls. Then they take as much of the raw beef as they want and cook it in the fondue pot for 2 to 3 seconds. Next they add the vegetables selected, which should warm up in about 1 minute. Finally, using a slotted spoon, diners transfer their beef and vegetables from the fondue pot to their bowls and then ladle on some hot broth and a spoonful of the sauce.
*Asian sesame oil and prepared wasabi are available at Asian markets and some supermarkets. You can order fresh wasabi from www.freshwasabi.com or make your own from wasabi powder (a mixture of horseradish powder, mustard powder, cornstarch, and artificial color) which is available on line from Gourmetsleuth.com