Combine the diced fish with lime juice in a non-reactive bowl, ensuring the fish is fully submerged. Cover and refrigerate for 15-30 minutes to ‘cook’ the fish.: When you first mix the fish with the lime juice you will notice the liquid glossing the pieces, and after a few minutes the edges begin to turn opaque, signaling that the proteins are tightening. I always use a glass or ceramic bowl because metal can react with citrus and alter the flavor. Listen for the quiet hush of chilled ingredients settling into the marinade, and watch the color shift as a visual cue the curing is working. This step is essential because the acid firms the texture and infuses flavor; rushing it will leave the fish overly soft and under flavored. A frequent mistake is leaving the fish in the juice too long, which makes it tough, so check texture at about 15 minutes and taste to judge doneness.
Mix in the red onion, tomatoes, jalapeño, and cilantro with the marinated fish, then season with salt to taste.: As you fold these components into the marinated fish , you will hear gentle rustling and see bursts of color as the tomatoes and herbs mingle. The aroma brightens immediately, with herbal notes from the cilantro and a sharp onion perfume cutting through the citrus. This is where layers of flavor assemble, so add the red onion and jalapeño gradually to control heat and bite. I recommend tasting before salting heavily because the acids and juices concentrate as they sit. A common error is over stirring which can bruise the tomatoes and make the mixture watery, so fold gently until just combined.
Gently fold in the diced avocado just before serving to maintain its texture and freshness.: The moment you add the avocado , the bowl changes from zesty and bright to luxuriously smooth in places, with soft green cubes punctuating each spoonful. I always add it at the last second to prevent browning and to ensure it keeps its clean shape. Use a light hand when folding, so the avocado remains intact and the marinade does not become creamy and homogenized. Smell for the subtle sweet earthiness of the avocado as it meets the citrus, and stop folding once the pieces are evenly distributed. A typical misstep is mixing the avocado in too early, which both colours the marinade and flattens the textures.
