Preheat the oven to 180°C (350°F), Gas Mark 4. Place the fish in a large bowl and sprinkle over the Cajun seasoning. Gently shake the bowl to coat the fish pieces in the seasoning.
Arrange the taco shells upright in an ovenproof dish and warm in the preheated oven for 5 minutes.
Meanwhile, heat the oil in a large frying pan and cook the fish pieces for 2–3 minutes on each side until cooked through. Carefully transfer the fish to a bowl and drizzle with the lime juice.
Fill the taco shells with the lettuce and top with the fish. Scatter the coriander on top and serve immediately with the tomato salsa and guacamole.