Line a baking sheet with foil, then drizzle and rub all over the surface with a little oil. Rub the fish with oil, salt and pepper and place on the baking sheet.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, uncovered, for 15–20 minutes or until the fish is cooked. Remove the heads (if left on) and all the bones. Break the fish, including the skin, into bite- sized chunks and put these in a mixing bowl.
Mix with the lemon grass, shallots, spring onions, ginger, kaffir lime leaves, mint leaves, soy sauce, lime juice and chillies. Taste and adjust the seasoning. Divide among 4 serving plates next to a pile of mixed salad leaves.