Cut the fish into small pieces: You can dice it or leave it in pieces up to ½ inch square, but remember that the larger the pieces the longer it will take to “cook.”.
Salt the fish, then cover with the citrus juice in a non-reactive (glass preferably) container with a lid. Add the sliced onions, jalapenos, lime zest and sugar.
Chill in the fridge for about 4 hours. When you take it out of the fridge add the cilantro and stir. The "cook" time is the time in the fridge. The acid will "cook" the fish to a nice consistency.
To serve: place ceviche in a glass bowl and scoop using a plastic serving spoon. DO NOT use a metal spoon because the acid will react with the metal.
Ceviche is so sharp and acidic it cries out for beer and tortilla chips as an accompaniment -- although you won't find tortilla chips in Peru. In Peru, you will most often find this served with potatoes, either sweet or white.