This recipe serves 8 appetizer portions of ceviche (also spelled Seviche). To serve 8 as a main course, marinate 8 fillets and increase other ingredients accordingly.
Rinse, dry, and dice tilapia fillets. Place in a large, shallow dish and toss with lime juice. Cover and allow fish to marinate in refrigerator for 4 hours.
Drain fish and discard juice. Combine fish with pepper flakes, cilantro, garlic, and tomatoes. Squeeze juice of ½ a lime over fish mixture.
Line salad plates or bowls with lettuce. Gently toss fish to mix well, then pile on top of lettuce leaves.
Thinly slice remaining lime half and use slices as garnish. Serve immediately.
The acidity in the limes affects the proteins in the fish much the same way heating does. Raw fish or shellfish becomes firm and opaque, as well as flavored by the juice. Any acidic juice or combination of juices can be used to make ceviche, and many South American countries have signature marinade recipes. Marinating doesn't kill parasites (although freezing raw fish does). If you're squeamish about eating uncooked fish, go ahead and cook the fish, then marinate it briefly for the ceviche.