In a large bowl, mix together the tomatoes, mango, orange, red onion, and jalapeño (if using).: The moment you toss the chopped grape tomatoes , diced mango , segmented orange , finely diced red onion , and chopped jalapeno you will notice a riot of colors and scents, citrus and tropical sweetness mingling with a faint onion aroma. Use a roomy bowl so juices can mingle without spilling, and gently fold rather than smash to preserve texture. At this stage you want each element to remain distinct, so aim for uniform pieces that distribute evenly. A common mistake is over mixing which makes the fruit release too much juice, watering down the final balance, so be delicate.
Stir in the shrimp and season with salt.: When you add the chopped shrimp , you'll feel the mixture firm up visually as the seafood integrates. Sprinkle the sea salt evenly rather than dumping it in one spot to ensure balanced seasoning. The salt heightens the natural brininess of the shrimp and enhances the sweetness of the fruit. Avoid adding too much salt early on, you can always adjust after the lime has done its work, and oversalting at this stage can mask delicate flavors.
Add the lime juice and toss.: Pour the fresh lime juice over the bowl so the acid bathes each piece, then toss gently to coat. The scent of citrus will become prominent, and you may notice the surface glisten as the juice spreads. This acid is what cures the shrimp , changing its texture and flavor, so ensure there is enough juice to contact all pieces. A common slip is skimping on juice, which leaves some shrimp undercured and inconsistent in texture.
Cover and refrigerate for 20 minutes. When the shrimp is done 'cooking' in the lime juice, it should have an opaque color (more white or pinkish, instead of transparent).: As the ceviche chills, the aromas settle and the texture transforms; you will see the shrimp go from translucent to opaque, signaling that the acid has firmed the proteins. The mixture should cool and the flavors will meld, giving you a more cohesive bite. If you leave it too long, the fruit can break down and the shrimp may become overly firm, so test early if you prefer a softer texture. A typical mistake is assuming refrigeration equals cooking time, but visual cues like opacity are the reliable indicator.
Just before serving, gently stir in the diced avocado and cilantro, if adding.: Right before plating, fold in the diced avocado and roughly chopped cilantro so the avocado stays vibrant and intact. The creamy texture of the avocado mellows the acidity and makes the mouthfeel luxurious, while the herbaceous cilantro injects a final burst of freshness. Stir carefully to avoid mashing the avocado , and add a small pinch of salt to taste if the mixture needs brightening. A common error is adding avocado too early which causes browning and a less appealing presentation.