Cut halibut to bite-sized pieces and place in shallow dish. Cover with 1 cup fresh lime juice and set aside for 20-25 minutes.: Right away you will notice the cool, silvery sheen of the raw halibut as you slice it into consistent, bite sized cubes. The pieces should be similar in size for even curing, and the flesh will feel firm yet yielding under the knife. If the fish smells overly fishy, do not use it, as freshness matters for both safety and flavor. A common mistake is cutting pieces too large, which prevents the lime from penetrating fully and leaves the center too raw when you finish.
Meanwhile mix together onion, tomatoes, serrano pepper, cilantro and salt to taste.: As the limes hit the fish, you will see the flesh begin to turn opaque and firmer around the edges, which signals the acid is denaturing the proteins. The aroma of citrus will become dominant, bright and almost floral, and the dish will carry a clean, sharp tang. I recommend tasting a small piece at 20 minutes to check texture, because overcuring can make the fish tough and rubbery. Avoid the mistake of leaving the fish in juice for too long, which shrinks and dries the meat.
Drain lime juice from halibut (discard lime juice). Add halibut to the pico-mixture and gently fold together.: While the fish cures, combine the chopped red onion , seeded tomatoes , finely diced and seeded serrano pepper , and chopped cilantro in a bowl. The onion will contribute a crisp little crunch, the tomatoes add juicy sweetness, and the pepper brings a warming heat that develops as it sits. Add salt sparingly at first because it will concentrate as flavors meld. A troubleshooting tip is to let this mix rest ten minutes so the flavors marry, but do not over-salt early, which can make the overall mix flat.
Squeeze the juice of 1 more lime on top of ceviche and mix together. Enjoy!: When the curing window is up, carefully drain the used lime juice from the shallow dish and discard it. You will notice the fish has become opaque and slightly firm, with a faint citrus scent. Draining prevents the ceviche from becoming too acidic or soggy when combined with the pico style mixture. One mistake to avoid is leaving the fish sitting in the drained juice, which can reintroduce excess acid and alter the intended balance.
Add halibut to the pico mixture and gently fold together: Now add the cured halibut to the bowl with the vegetable mix and use a gentle folding motion to combine, preserving the fish pieces and avoiding mashing. You will hear a soft movement as textures mingle, and the combined aromas should be bright and layered. The folding technique keeps the fish intact and ensures even distribution of veggies without breaking the pieces. A common error is stirring too aggressively, which will bruise the fish and make the presentation sloppy.
Squeeze the juice of 1 more lime on top of ceviche and mix together: Finish by squeezing an additional lime over the assembled ceviche to add a final, aromatic brightness that ties everything together. This last splash of citrus sharpens flavors and brings a lively lift to the herbs and pepper. Mix once gently so the juice coats everything evenly, then taste and adjust salt if needed. Avoid over squeezing more limes than called for, because extra acid can overpower the delicate fish texture.
Enjoy: At this point the ceviche should smell fragrant and look vibrant, with glossy pieces of cured halibut nestled among colorful veggies. Serve immediately for the best texture, noting the pleasing contrast of tender fish against crisp onion and herbacious cilantro . If you must hold it, refrigerate for a short time, but remember that extended standing will continue to firm the fish and change the mouthfeel. The most common mistake is leaving it too long before serving which reduces the refreshing quality that makes ceviche special.