Place the pieces of cod in a pan large enough to hold them in a single layer. Just cover with cold water and bring slowly to the boil over a moderate heat. Poach gently for about 10 minutes or until the fish is tender.
Meanwhile, melt the margarine in a pan and sauté the onion for 5—7 minutes, until soft but not coloured. Add the stock, orange juice, vinegar, soy sauce and sugar to the pan and stir well to combine.
In a small bowl, blend the cornflour with a little water to make a creamy paste and add to the sauce. Bring to the boil, stirring, until the sauce is thickened. Simmer for 3—5 minutes.
When the fish is cooked, lift the cod portions out of the pan with a slotted spoon and place in a warmed serving dish. Pour the sauce over the fish. To serve, garnish with strips of orange rind and spring onion curls.