In a glass bowl, combine the oysters, lime juice, onions, tomatoes and chile. Toss to combine. Cover and refrigerate until oysters have slightly "cooked," 1 to 1 1/2 hours. (Note: If oysters are allowed to marinate too long, they will become tough and rubbery.)
Before serving, mix in mint. Season, to taste, with salt and freshly ground black pepper.
Arrange 1 lettuce leaf on each of 4 plates. Arrange the ceviche inside the leaves and serve.
Yield: 4 servings