Cut the fish into small squares, place into a non-reactive bowl (glass is best), cover with water and salt. Cover and let rest in the fridge while you prepare the rest of the ingredients.
Rinse/remove the salt water. Keep the fish in the glass bowl/container and add the lime juice, crushed garlic, hot peppers, cilantro sprigs, and salt. Cover and refrigerate for 3-4 hours.
Soak the sliced onions or shallots in salted water, rinse well and drain.
Marinate the sliced red onions or shallots with lime juice and salt for ~15-30 minutes.
Remove the crushed garlic, hot peppers and cilantro springs from fish and lime mix.
Combine the fish with the marinated onions/shallots with chopped cilantro, and oil. Taste and adjust salt/lime/cilantro/etc to taste.
Serve with tostado, popcorn, chifles or patacones.