In 1963 I was invited to join friends for a visit to Mexico. There were five of us in a four-passenger plane and our former-crop-duster pilot chose not to to use the recommended oxygen as we landed late at night in Mexico City. You may assume we had quite a few adventures on that trip.
The guest rooms of the hotel in Oaxaca surrounded a beautiful square patio full of luxurious greenery. The heavy handmade furniture was gradually (and proudly) being upgraded to pink plastic from the States.
We arrived in time for lunch. I had never heard of ceviche - and have never forgotten it. I ordered it wherever it was offered and loved every rendition.
A pound, more or less, of white fish such as halibut (my favorite but expensive), flounder or snapper or trout. A brief time in the freezer will make it easier to cut into 1/4 inch dice. Cover completely with freshly squeezed lime juice. After 2 or 3 hours, the fish will be opaque, "cooked" by the lime juice. Drain off and discard the lime juice. Refrigerate the fish.
In a glass or stainless bowl combine finely chopped red onion, minced garlic, half a jalapeño more or less, probably less - no seeds - very finely chopped, two or three tablespoons of catsup, some sliced pimiento-stuffed olives, a teaspoon of sugar. Add a diced tomato, salt and the fish. Mix gently. Just before serving add a diced avocado, a bit more fresh lime juice, a splash of olive oil and a lot of fresh cilantro.
Bay scallops or diced sea scallops work well also.