Cook the potatoes in lightly salted boiling water for 10 minutes until just tender, drain.
Meanwhile heat the oil in a large pan and cook the onion, garlic and leeks for 5 minutes until softened.
Add the tomatoes and soy sauce and season to taste, then cover and simmer for 5 minutes.
Add the potato pieces and fish to the pan. Cover and simmer for about 10 minutes until the fish is cooked.
Sprinkle with the parsley, then serve with crusty bread to mop up the juices.
As an alternative to cod use a mix of fish.