Heat the oil in a large frying pan over medium-high heat. Season the fish with salt and place in the frying pan. Fry on both sides until fish is browned and cooked through, about 5 minutes per side. Remove from the frying pan and set aside.
Fry the onions and garlic in the same frying pan over medium heat until translucent. Add the peppercorns, allspice berries, bay leaves and red chilli. Pour in the vinegar and water and bring to the boil. Stir in the dark brown soft sugar until dissolved. Season with curry powder, turmeric, cumin and coriander. Taste and adjust the sweetness if desired.
Layer pieces of fish and the pickling mixture in a serving dish. Pour the liquid over until the top layer is covered. Allow to cool then cover and refrigerate for at least 24 hours before serving.
Alternatives to cod:
Try this recipe with inexpensive fresh coley or pouting. They both belong to the cod family but are much cheaper and available in larger supermarkets or fishmongers.