Wash, dry and thinly slice your fish fillets into 1/4-inch pieces. Place in a non-metallic bowl or shallow dish. Stir together the lime juice, fish sauce, spring onion and salt. Pour over the fish, stirring slightly to ensure all pieces of the fish are coated in the liquid. Cover with plastic wrap and keep in the fridge for 45 minutes to 1 hour.
Separate the leaves of the gem lettuce, being sure to keep each leaf in one piece. Depending on their size, 3-4 gem lettuce heads should yield about 2 dozen leaves. Wash and dry well. Place each leaf curved side up on a large serving platter. Once the fish has rested for one hour, remove it from the fridge and divide evenly among the lettuce leaves.
Top each lettuce wrap with a sprinkle of sesame oil, chili, cilantro, mint, spring onion and sesame seeds. If you like, you can also spoon a little of the marinating liquid into each wrap. Serve immediately.
Only fish to avoid in a ceviche recipe is an oily fish such as tuna, sardines or mackerel.