Halve, then thinly slice the onion. Place in the bowl.
Finely chop the green chillies, and add them to the bowl.
Finely chop the Peruvian black mint leaves and thin stalks. Use the thicker stalks. Add to the bowl.
Chop the tomatoes into tiny pieces and add to the bowl, including any juices.
Squeeze the lime juice onto all the chopped ingredients in the bowl.
Add a pinch of salt and a dash of freshly ground black pepper and stir everything up to mix.
Now, we do the fish. Remove any dark meat on the underside of the fish, we want white meat for the cleanest taste. Then cut up the fish into little cubes, about 1cm (under half an inch) all across.
Add the fish to the marinade and mix well (with a gentle touch) with 2 spoons, lifting up the ingredients, being careful not to squash the fish and tomatoes.
Taste, and add more salt if necessary.
Divide on 4 plates, bowls or glasses, with the sliced avocado alongside, and an extra sprinkle of Huacatay and black pepper.