For the broth and vegetables:
7 cups chicken stock, preferably homemade (page 338)
One 3-inch piece fresh ginger, peeled and thickly sliced
4 garlic cloves, peeled and smashed with the side of a knife
8 scallions, white and 1 inch of the green parts, coarsely chopped
2 tablespoons soy sauce
1 tablespoon Asian (toasted) sesame oil*
1 small Napa cabbage, cored and thinly shredded
6 carrots, thinly sliced
2 large red bell peppers, cut into 1-inch pieces
1/2 pound shiitake mushrooms, stems discarded, caps sliced
1 1/2 pounds boneless shell or sirloin steak
1/2 pound sugar snap peas, blanched in boiling water for 30 seconds, drained, and shocked in ice water

For the sauce:
1/2 cup sour cream
1 teaspoon prepared wasabi*
1 tablespoon snipped fresh chives
1/2 teaspoon dry mustard
Kosher salt and freshly ground black pepper to taste