In a blender or food processor, add all the ingredients for the coconut sauce, including lemongrass. Blend well. Taste for seasoning and add more soy or a pinch of salt.
In a skillet over high heat, add the olive oil over and sauté the prawns and the vegetables. Toss until done. Season with salt and pepper.
Gently heat the sauce, do not boil it as it will separate. When hot pour over the prawns and vegetables.