Seviche is extremely popular throughout Mexico, the Caribbean, and other parts of Latin America, and while it has many variations, it's basically a simple fusion of seafood and citrus juice, with a few other ingredients and spices. Seviche is frequently served as an appetizer or even as a light meal.
There's really no mystery to its preparation - when the acid of the citrus comes in contact with the fish, chemical reaction occurs, similar to what happens when the fish is cooked, and the flesh becomes opaque and firm. Though many people refer to the juice as "cooking" the fish, this is technically incorrect (and was probably thought up to coax squeamish gringos into trying it).
In Mexico, what we call Key limes are the principal citrus of choice. Here in the United States, many cooks use a mixture of lemon and lime.
Seviche can include a mixture of saltwater fish, scallops or other shellfish, squid or octopus. Onion, chiles, tomato, pepper and cilantro - and of course, citrus juice - are typical ingredients.
Most recipes say to marinate the fish for at least four hours to "cook" the fish all the way through. Longer marinating can cause the citrus to overpower the delicate flavor of the fish.
But if there is one absolute rule when preparing seviche, it is this: the fish must be absolutely fresh.
Cut the fish into 1/2-inch cubes. Place in a small glass dish and cover with 1 cup key lime juice. Make certain that the fish is completely covered, adding more juice if necessary. Cover tightly and refrigerate for 4 hours. Drain, discarding the liquid.
Combine the oil, orange juice, 2 Tbsp lime juice, chiles, tomatoes, onion, and garlic and add to the fish. Continue to marinate in the refrigerator for another hour. Remove from the refrigerator and add salt and pepper to taste.
Serve with fresh, crisp tortilla chips.