This soupy style of ceviche, found in Ecuador's coastal regions, is made to order at beachside carts (the carretas), usually with native black clams. We've used prawns here - clams would also work well in the mix. Unlike most ceviches, where prawns are added raw to the acidic juices of the marinade to "cook", here they are lightly boiled first and the stock is used in the marinade, along with a good splash of lime juice. The prawns can be chopped so the ceviche can be scooped up with fried tortillas or large corn chips. This ceviche, a great appetiser for a dinner party, is often served with tomato ketchup, which seems like an unusual condiment, but the sweetness of the sauce balances out the acid in the marinade.