Remove any outer leaves from the leeks and slice very finely. Line a bowl with kitchen paper and sprinkle over some salt. Put the celery slices in the bowl to purge for about 20 minutes.

Slice the fish fillets very thinly and put in a large plate. Drizzle over some extra virgin olive oil (about 3 tbsp), the white wine vinegar and sprinkle over some salt. Cover with cling film and let marinate for about 30 minutes in a cool place or in the fridge.

Blanch the tomato, remove the seeds and chop finely. Chop the parsley very finely too. Drain the capers, chop finely and set aside. Put the gelatine in 20ml water and let stand for about 10 minutes until it softens. Pour the contents in a small pan and add a further 40ml water and heat over a low heat.

As soon as it has melted completely, remove from the heat and set aside.

Drain the fish but reserve the marinade. Mix the marinade with the gelatine mixture, tomato, capers and parsley. Line a 1.5l terrine with cling film, and line the base of the terrine with a layer of leek slices dipped in the gelatine mixture. Top with a layer of tuna dipped in the gelatin and tomato mixture. Continue doing so alternating layers until you have used up all the ingredients. Cover with the remaining gelatine and tomato mixture and refrigerate for not less than 4 hours.

To serve, put the terrine in hot water for a minute or two and turn out onto a serving plate.