Wash the prawns well. Leave in a sieve for a while, then pat dry. Put in a bowl. Add the cayenne pepper, pepper, salt and chillies. Mix well. The prawns may be covered and refrigerated until needed.

Put the oil in a karhai, wok or frying pan and set over a medium—high heat. When hot, put in the mustard seeds. As soon as they begin to pop, a matter of seconds, add the garlic and stir once or twice. Quickly put in the prawns and curry leaves. Stir a few times. Add the grated tomato. Stir a few times. Turn the heat down to medium—low and let the prawns cook gently, stirring just until they turn opaque, a matter of 2 or 3 minutes. Serve immediately.