Start by cutting the halibut into half inch cubes. The smaller the cut on any fish that you're turning into ceviche, the faster it will "cook" in the acid. Set the halibut aside in a small, non-reactive container (read plastic or glass, preferably plastic).
To start the ceviche: core, peel, and dice your tomatoes just like you would with a tomato concassé. If you're unfamiliar with this technique, start by removing the core of the roma tomato and then cut an X in the top of the Roma, extending it down to the core, making sure to only cut through skin, not the flesh. Drop into boiling, salted water for about 30-60 seconds and then rapidly chill in an ice bath.
When removed from the ice bath, you'll notice that the skin on the tomato has started to peel back slightly. Gripping the skin between your knife blade and your thumb, peel it back, removing completely. From here, simply seed and dice.
Next: peel, seed, and dice an English cucumber and add it to your tomatoes in an appropriately-sized mixing bowl. Add to this tomato/cucumber mixture the diced red onion, using the basic onion dicing technique.
Add the diced cucumber, onion, and tomato to the halibut and mix thoroughly to achieve an even dispersion.
Cover your ceviche mixture with lime juice and add in the Clamato and ketchup. Yes, ketchup. Ketchup is actually common in the ceviche recipes found in Northern Mexico, obviously influenced by their northern neighbor's infatuation with salsa el american.
The Clamato, a mix of clam juice and tomato juice, helps round out the flavors of the ceviche liquid as well. Add in some salt to taste, and a little sugar, to counterbalance the acidity of the lime juice and you're good to go.
Allow everything to marinade in the ceviche liquid for at least two hours before serving, but no longer then four. When the ceviche is ready to serve, you will notice that the flesh has turned opaque and has become firm, almost as if it were cooked (which technically speaking, it is).
When serving the ceviche you can simply place it in a bowl and serve it alongside lime wedges, lightly chopped cilantro, and tortilla chips.