1. Remove any strings from the skeins of bean threads. Place the bean threads in a small bowl and add enough very hot tap water to cover. Let stand while you prepare the soup.
2. Holding the knife at a 45-degree angle, cut the steak across the grain into thin slices (holding the knife at this angle makes wider slices). Cut the slices into pieces about 3 inches long.
3. Heat 1 tablespoon of the oil in a large saucepan over high heat until very hot. In batches, without crowding, add the beef and cook, stirring occasionally, until it loses its pink color, about 11/2 minutes. Transfer to a plate.
4. Add the remaining 1 tablespoon oil to the saucepan. Add the shallot, ginger, and garlic and stir-fry until the shallot softens, about 1 minute. Add the bok choy and stir-fry until wilted, about 2 minutes. Add the broth, 2 cups water, the sherry, soy sauce, brown sugar, and chili paste and bring to a boil.
5. Meanwhile, drain the bean threads and return to the bowl. Using kitchen scissors, snip through the bean threads a few times to make shorter lengths (this makes them much easier to eat with a soupspoon). Stir the beef and bean threads into the soup. (The soup can be cooled, covered, and refrigerated for up to 2 days, or frozen for up to 3 months.)
6. Ladle the soup into deep soup bowls and serve hot.
Chinatown Shrimp and Spinach Soup
Substitute 12 ounces peeled, deveined medium shrimp for the beef and stir-fry until it turns opaque, about 3 minutes. Substitute 8 cups loosely packed baby spinach (about 7 ounces) for the bok choy.
There are countless brands of soy sauce, which is made from a brew of fermented, aged, and distilled soybeans. The problem is that each producer has its own recipe, and the quality varies enormously (I have ruined a recipe by using a soy sauce that was too strong). To confuse matters further, there are three grades: light (not to be confused with low-sodium), medium (also known as thin soy sauce), and dark (with added molasses, and sometimes called superior soy sauce--which is not an indication of quality), as well as mushroom-flavored soy sauce. Asian cooks use each of these differently.
What's a Westerner to do? Supermarket soy sauce is usually a Japanese brand with reliable quality and flavor, so that's what I recommend. Traditional Asian cooks can argue that it is worthwhile to appreciate the differences among the three soy sauce varieties, but for everyday cooking, I am very happy with the supermarket brand.