For the salsa – in a blender, combine the fresh lime juice, green chilli, coriander and a pinch of salt and pepper. (If you don’t have a blender, chop the chilli and coriander finely and mix by hand with the lime juice.) Set aside.
For the garnish – cut the red onion in half and finely slice half circles. Put the sliced onion straight into iced water for five minutes then drain. Cut the cucumber in half horizontally and finely slice ribbons length ways.
Lay the prawns flat on the cutting board, and slice them in half along the vein line.
In a bowl, combine the cut prawns and the Aguachile salsa. Ensure all the prawns are coated in salsa. Leave to cure for five minutes.
With a big serving spoon (or your hands), top each tostada with the prawns, and garnish with the red onion, cucumber and two slices of avocado.