How to Make It
In a 1 1/2-quart glass or stainless steel bowl, combine the fish, lime juice and onion. Use enough juice to cover the fish and allow it to float freely; too little juice means unevenly "cooked" fish. Cover and refrigerate for about 4 hours, until a cube of fish no longer looks raw when broken open. Drain in a colander.
In a large bowl, mix together the tomatoes, green chiles, cilantro, olives and optional olive oil. Stir in the fish and season with salt, usually about 1/2 teaspoon. Add the orange juice or sugar. Cover and refrigerate if not serving immediately. Just before serving, gently stir in the diced avocado.
There are many ways to serve ceviche. Here are some of our favorites: Place the ceviche in a large bowl and let people spoon it onto individual plates to eat with chips or saltines; spoon the ceviche into small bowls and serve tostadas, chips or saltines alongside; or pile the ceviche onto chips or tostadas and pass around for guests to consume on these edible little plates. Garnish the ceviche with cilantro leaves before serving.