Warm the oil in a large wide saucepan, and throw in the scallions, garlic and ginger.
Stir everything around vigorously until the onions begin to wilt, and then toss in the prawns, stirring again. Cover the pan and let everything cook for about 5 minutes; then, remove the lid and pour in the sherry. Stir again, then cover the pan until the prawns are cooked through and no longer glassy in the middle. Taste; add a sprinkling of salt if you need it. Pour into a large shallow bowl, making sure you don't lose all the bits of onion, garlic and ginger, and take immediately to the table.