1 tablespoon cooking oil
6 scallions including green tops, chopped
4 cloves garlic, minced
2 teaspoons chopped fresh ginger
1/4 cup soy sauce
1/4 teaspoon dried red-pepper flakes
3 1/2 cups water
3 1/2 cups canned low-sodium chicken broth or homemade stock
4 carrots, cut diagonally into 1/4-inch slices
3/4 pound napa (Chinese) cabbage (about 1/2 head), leaves shredded (about 3 cups)
3/4 teaspoon salt
1 small head bok choy (about 3/4 pound), stalks halved lengthwise and cut crosswise into 1/2-inch pieces, leaves shredded
Grated zest of 1 lemon
2 tablespoons lemon juice
1/4 pound vermicelli
1 1/2 teaspoons Asian sesame oil