Preheat barbecue or indoor grill to medium-high. Place the first 7 ingredients in a small pot. Bring to a simmer, stirring to blend all ingredients together. Simmer 2 minutes, and then cover and reserve on low heat.
Use paper towel to remove any excess moisture on the prawns. Brush the prawns with the oil. Grill 1 to 2 minutes per side, and then set 2 on each of four serving plates. Spoon the sauce over top, sprinkle with green onion and enjoy.