Put the diced fish in a small stainless steel or glass bowl and cover with the lime juice. Be sure the fish is completely covered; add more lime juice if needed to do so. Stir in a generous half teaspoon of salt. Cover and refrigerate for at least 1 hour and preferably 2 or 3 hours. Thirty minutes before serving, stir in the serrano, onion and cilantro.
Season lightly with salt, add a sprig of cilantro and a handful of chips to each plate and serve immediately.
2 tablespoons chopped fresh cilantro