1. Cover the dried mushrooms with 1 cup of warm water and let them soak for 30 minutes. Drain them and save the water. With a sharp knife, cut away the tough stems, then slice the caps into strips ¼ inch wide.
2. In a heavy 3- to 4-quart saucepan, combine the reserved mushroom water, the stock and onions. Bring to a boil over high heat, reduce the heat to low and simmer for 3 or 4 minutes, until the onions are soft.
3. Stir in the crab meat, asparagus and mushroom strips and bring the soup to a boil again. Then reduce the heat. Combine the cornstarch with ½ cup of cold water, stir well, and pour it into the soup. Stirring constantly but gently, simmer until the soup thickens and is clear.
4. Taste for seasoning and serve at once from a heated tureen or in individual soup plates. Just before serving, crumble the salted egg yolk into small bits and sprinkle the yolk and scallions on top of the soup.
