You don’t have to serve this recipe with fish, this is really a recipe about using up old lettuce, and the gratin makes a lovely bed for fish, chicken, jack fruit, or marinated tofu. This sort of recipe works for any more solid leafy greens that have a bit more “back bone” or structure to them, like iceberg lettuce, romain lettuce, chicory, pak choi or chard. Very young or soft salad leaves (the kind you tend to get in bags of mixed leaf salad) won’t hold up to a trip through the oven, so avoid using them.