Clean the horse mackerels and remove any bones.  Flake the fish into chunky pieces or cut it into strips depending on how large you want them to be but not too small (1 or 2 cm). Set aside in water with ice.
Julienne the onion and rinse it.
Mix the onion with the lime and the lemon juice, the minced chilli pepper, the parsley, the cilantro and a pinch of salt and pepper.
Add the fish and wait for 10 or 20 minutes.
Meanwhile, slice the avocado with the mandoline slicer. You can cover it with cling film and place it in the freezer to chill for a while to firm up.
Strain the mixture and serve it covered with a slice of avocado.