Preparing the crab

If you are beginning with a whole, live crab, either plunge it into unsalted boiling water for 1 minute or steam it for 5 minutes, until it stops moving. Clean as described in TECHNIQUE NOTES, crack it, and extract all the meat. Pick over the meat carefully, then put aside ½ pound. If you are using frozen crab, defrost it, break it into coarse bits, and pick over carefully to remove shells and cartilage. The crab may be prepared up to a day in advance. Seal airtight, refrigerate, and bring to room temperature before cooking.

Making the soup

About 10–15 minutes before serving, put individual soup bowls in a low oven to warm. Have all the ingredients within easy reach of your stovetop.

Set a heavy, non-aluminum stockpot over high heat until hot enough to evaporate a bead of water. Add the oil to the pot, swirl to coat the bottom, then reduce the heat to medium-high. When the oil is hot enough to sizzle a bit of scallion, add the scallion and ginger, adjusting the heat so they sizzle without scorching. When fully fragrant, about 10–15 seconds, add the ham or crab. Stir briskly to combine, about 10 seconds, then add the wine. Wait a split second for it to “explode” in an aromatic hiss, then toss the meat briskly several times and immediately add the stock. Stir to blend, add the corn, and stir again. Bring the mixture to a near-boil over moderate heat. Stir frequently and watch that the heat does not climb too high, lest the corn burn.

Reduce the heat to low, taste, and add salt as required. Stir the cornstarch mixture to recombine it, pour it evenly into the soup, and stir gently for about 1 minute until the mixture becomes glossy and slightly thick. Turn off the heat. Beat the egg whites lightly with a fork or chopsticks to break the gel. They will froth a bit, but do not beat to a foam. Add them to the soup in a thin, steady stream, pouring from a stationary spot about 6 inches above the pot. Stir gently once midway, then again after all the egg whites have been added, to bring the lacy threads to the surface.

Serve at once, ladling the soup into the heated bowls and garnishing each with a sprinkling of coriander or chopped scallion. Or, cover the pot and keep the soup warm over the lowest possible heat. It will keep nicely this way for an hour, if you need to cook in advance.

Leftovers keep well 4–5 days, refrigerated, and also survive freezing in remarkably good shape. Rewarm in a heavy pot over moderately low heat, stirring frequently.