For the ceviche: Place the fish pieces in a nonreactive bowl or shallow dish. Stir together the lime juice, fish sauce, spring onions and 1 teaspoon salt in a small bowl; pour over the fish, stirring gently to coat all the pieces. Cover with plastic wrap and refrigerate for 45 minutes to 1 hour.
For the lettuce wraps: Place each lettuce leaf, curved side up, on a large serving platter. Remove the fish from the refrigerator and divide it evenly among the leaves.
Top each lettuce wrap with a sprinkle of cilantro, mint, spring onions, red chiles, sesame oil and sesame seeds. If you like, you can also spoon a little of the marinating liquid into each wrap.
Avoid oily fish (such as tuna, sardines or mackerel) when making ceviche.