Pour the citrus juice into a bowl. Crush the chiltepin chiles, if using, and add to the juice, then add the diced red onion, oregano and garlic. Sprinkle a little salt over it all.
Slice the fish into small dice, making sure to slice any areas with strong sinew, mostly near the tail, thinly across the grain of the meat. Add to the ceviche mix. Chill this for 30 minutes, for the fish to marinate.
When you are ready to serve, add the remaining ingredients, stir well and serve cold with tortilla chips or on tostadas.
Notes
If you want some heat but don't have hot, dried chiles like the chiltepin chiles, simply mince a serrano, habanero or a couple jalapenos and add with the red onion.Don't like mango? Substitute pineapple chunks.