1. Bring the stock, lemon grass, red pepper, shallot, and scallions to a boil in a large nonreactive pot. Reduce the heat and simmer, skimming any foam from the surface, for 30 minutes.
2. Scrub the clams under cold running water. Rinse the bok choy, shake dry, and trim off the white ends. Slice crosswise into ¼-inch-wide ribbons. Leave straw mushrooms or honshimeji whole but stem, then slice shiitake caps, if using, ¼-inch thick. Trim the stem ends off the enoki and separate the clusters. Core the tomato, remove the seeds, and cut the pulp into ¼-inch strips.
3. Remove the lemon grass bundle from the broth. Bring broth back to a boil. Add the clams and straw mushrooms, cover, and reduce the heat to medium. Steam the clams until they open, about 3 minutes. Stir often to give all the clams a chance to open. Any that do not open should be discarded.
4. Stir in the bok choy, and cook over medium heat, stirring, for 1 minute. Stir in the enoki, and cook for 1 minute. Add the tomato strips. Season to taste with salt, top with the cilantro leaves, arid serve.
Variation: To serve over jasmine rice, spoon about 1 cup cooked jasmine rice into each of 2 bowls. Transfer the soup solids with a slotted spoon, top each serving with cilantro leaves, and spoon the broth over all.