Cut the fish into 1/2-inch pieces or slightly smaller.: You will notice the fish feels firm and slightly slick as you cut it, the clean ocean scent is fresh and not overpowering. Cutting into uniform 1/2 inch pieces ensures even marination, so each cube firms up consistently in the lime juice. Use a sharp knife to avoid tearing the flesh and keep the pieces similar in size so some do not over cure while others remain underdone. A common error is leaving large chunks which can remain too raw in the center and create an uneven texture.
To marinate the fish, place in a glass or plastic bowl and mix in the onion, black pepper and lime juice. Cover and refrigerate, stirring frequently, for 45 minutes.: As the lime juice hits the fish you will hear a soft hiss and see the flesh turn opaque at the edges, that is the acid starting to 'cook' the protein. Mixing with thinly sliced red onion and freshly ground black pepper distributes flavor through the pieces. Use a non reactive bowl to prevent metallic notes. Stir gently to coat without breaking the cubes. If you skip stirring, pockets of fish may remain uncoated and not develop the proper texture.
When ready to serve, add about a teaspoon of salt to the ceviche and mix well (the saltiness of the juice will dissipate as the fish absorbs the flavors.): During refrigeration you should notice the fish progressively firm and the liquid becoming cloudy with extracted proteins, which is normal. Stirring every 10 to 15 minutes promotes even curing and keeps the onion from settling on the bottom. Keep the bowl chilled the whole time, as warm temperatures can accelerate breakdown and increase food safety risk. A frequent mistake is leaving the ceviche uncovered or unrefrigerated which can lead to uneven curing and safety issues.
Scoop the ceviche onto a smallish, deep platter, then dot with avocado and cilantro leaves. Serve as an appetizer to eat with a fork, or use tortilla chips for scooping.: After marination, add the salt gradually and taste, because the perceived saltiness changes as the fish soaks up the lime. The salt will lift the flavors and balance the acidity, and the texture will feel fuller in the mouth. Mix gently so you do not mash the pieces. Over salting is an easy misstep; add a bit, taste, then adjust if needed.
Scoop the ceviche onto a smallish, deep platter, then dot with avocado and cilantro leaves: Plating reveals the ceviche's translucence turning to opaque, and the bright green of the diced avocado creates visual contrast while adding a cooling mouthfeel. Dotting the plate instead of mixing the avocado in prevents it from breaking down and turning mushy. Finish with the lightly packed cilantro leaves for fragrance. If you stir the avocado in too early it can become gummy and lose its texture.
Serve as an appetizer to eat with a fork, or use tortilla chips for scooping: The final sensory moment is the first bite, where you get citrus brightness, pepper warmth, crunchy onion, creamy avocado, and herbaceous cilantro. Choose forks for a refined presentation or sturdy tortilla chips for a casual crunch. Keep extra chips or spoons nearby so guests can serve themselves. A misstep is serving with flimsy chips that break at the first scoop, which distracts from the experience.