Bring 1½ litres water and 2 tsp salt to the boil in a large saucepan, add prawn shells, return to the boil, then reduce heat and simmer until stock is flavoured (2-4 minutes). Strain, discarding shells, then return stock to pan, add prawns and simmer until opaque (1-2 minutes). Drain, reserving 250ml cooking liquid, and cool.
Combine citrus juices and jalapeño with prawns and stock and refrigerate to chill (1-2 hours).
Place onion, 250ml cold water and 3 tsp salt in a bowl and refrigerate to soften (5-7 minutes), then drain.
Stir coriander into ceviche, top with diced tomato, avocado and onion, and serve with corn chips and ketchup.
