Check the fish is impeccably fresh and cold. Cut it into even half inch cubes; uneven pieces cure unevenly.
Toss the cubes with the salt and the lime juice in a non-metal bowl, making sure the fish is fully submerged. Refrigerate.
{'Cure 20 to 30 minutes for half inch cubes, stirring once. Cut a piece in half': 'it should be opaque most of the way through with a faint translucent line at the centre.'}
Drain off most of the lime juice, keeping a few spoonfuls. Draining is what stops the curing.
Fold in the red onion, tomato, cucumber, serrano and cilantro.
Taste for salt and lime, then fold in the avocado last so it stays in distinct pieces.
Serve cold, within the hour, with tostadas, tortilla chips or saltines.