Heat the oil in a frying pan over medium heat. Place onion in the frying pan. Reduce heat to low, and cook, stirring often, 15 minutes, or until soft and brown.
Mix the garlic and ginger into the frying pan. Cook 1 minute. Mix in cumin, coriander, cardamom, turmeric and salt. Stir in the green chillies, fresh coriander, lemon juice and tomatoes with juice, scraping up any brown bits from the bottom of the frying pan. Bring to the boil. Reduce heat to low, cover, and simmer 20 minutes. If you like, the sauce may be set aside for a few hours at this point to allow the flavours to blend.
Return the sauce to a boil, and place cod in the frying pan. Reduce heat to low, and cook 15 minutes, or until fish flakes easily with a fork.
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.
Quick and simple to make. Lovely, fresh, Thai like flavours. Used haddock fillets and the juice of a whole lemon rather than just a tablespoon. Also saved some of the chopped fresh coriander to sprinkle on top to serve. A winner! - 13 Jul 2012