In a blender or food processor, combine the lime juice, garlic, cilantro, chiles and 1 scant teaspoon salt. Process until smooth.
Scoop the fish into a large bowl. Pour the lime marinade over it and let it “cook” in the lime juice to suit your own taste: you can eat it right away (Peruvian-style) if you like raw fish or let it “cook” for an hour or two if you like it more well-done; cooked shrimp need only a few minutes to soak up the flavor. (It takes about 4 hours to “cook” fish well-done in lime juice; if that is your desire, add the cilantro, chopped, just before serving to preserve its fresh color.)
Pour off half of the marinating liquid and set aside. Toss the avocado with the fish, then taste and season with additional salt if you think necessary.
Divide the lettuce between 4 dinner plates. Scoop a portion of the ceviche into the center of each arrangement. Sprinkle with chopped green onion or cilantro. Drizzle some of the reserved lime marinade over the lettuce and you’re ready to serve.
Ceviche Riffs (Some for the Adventurous): Practically any edible piscine morsel can be made into ceviche: squid and baby octopus are favorites—I simmer both of them until tender (usually 20 to 30 minutes) before marinating them. Scallops are wonderful raw and barely marinated, as are sardines. Around Guadalajara, the fish markets offer ground fish for making ceviche to pile on little tostadas—it cooks through in a matter of minutes; if that appeals, look for ground fish for fish cakes in your local fish market. Avocado is just a start when it comes to add-ins: tomato, red or white onion, olives, cooked cactus (nopales), roasted poblano chile—those should get your creative juices flowing.