Combine the lime and lime juice in in a small mixing bowl. Place the mackerel fillets in a shallow dish and pour the citrus juice over them. Cover and marinate for 45 minutes in the fridge. Remove from the fridge and cut the mackerel into strips.
Transfer the strips of mackerel to a large mixing bowl and toss with the apple, celery, chopped tomato, chopped herbs, dried chilli flakes and seasoning. Add the extra-virgin olive oil and toss well again.
Spoon into serving glasses. Serve cold or chilled.