Thoroughly mix the cumin, cayenne pepper, lime juice, Tabasco and oyster sauce in a bowl.
Heat the butter in a small frying pan and fry the onions and garlic for about 5 minutes until just turning golden. Add the prawns and stir fry for 2 – 3 minutes until pink.
Put the contents of the pan into the bowl with the sauce and mix thoroughly. Serve on a bed of lettuce with the tomatoes and sprinkle over the fresh coriander.