Chilca-style fish soup

A small town to the south of Lima called Chilca has contributed the chilcano to Peruvian cooking, a fish soup enriched with lime and chilli. Sometimes it is made with the heads of the fish (an economical version once almost compulsory for students), at other times with the whole fish or with shellfish. For the soup to be a genuine chilcano it is important to make it with only one kind of fish.