Cut fish fillet into small cubes. Squeeze out juice from limes. Mix fish with lime juice and set aside in refrigerator for at least 10 minutes.
Cut onion into slices. Dice Serrano pepper. Remove cilantro leaves from sprigs and cut into strips.
When fish has ‘cooked’ in lime juice and turned slightly white, add onion, serrano pepper and cilantro. Mix ingredients well and add salt to taste.
Serve immediately with corn tortilla chips.