Heat the oil in a large frying pan over medium heat, and fry the chilli, leek and garlic until tender and lightly browned. Season with salt and pepper.
Stir in the chicken stock and tomatoes and season with cumin. Bring to the boil. Reduce heat to low.
Place the choice of fish into the mixture. Sprinkle with lime juice. Cook 15 to 20 minutes or until the fish is easily flaked with a fork. Wrap in warmed corn tortillas to serve with the suggested accompaniments above.