Mix all the ingredients for the sauce. Cover and refrigerate until needed.
When you are ready to eat heat the oil in a large frying pan over a medium high heat. When hot, put in the mustard seeds and , as soon as they begin to pop, the garlic.
Stir once and add the prawns. Stir and fry until they just turn opaque, sprinkling with salt and pepper.
Pour in the sauce and stir. As soon as the sauce starts bubbling the dish is ready to be served.