Heat a wok and add the oil. When it is very hot, add the clams and stir-fry them for about 30 seconds.
Add the remaining ingredients. Let them simmer about 5 minutes.
As the clams open wide, remove them one by one to a serving platter with long chopsticks. As you remove them, tip them to drain all the juices into the wok.
Any clams that fail to open should be discarded. They were dead before they were cooked. (If the beans and juice from the clams don’t thicken the sauce sufficiently, add the cornstarch dissolved in water.) Taste the sauce. During the long simmering, it can be reduced and become excessively salty. If it is too salty, thin it with water or more chicken broth. Pour the sauce over the clams and sprinkle the dish with chopped scallion.