Preheat oven to 190°C/375°F/Gas Mark 5.
Put the Tomato and Onion Cook-in-Sauce in a shallow ovenproof dish.
Wash the cod steaks and place in the dish.
Chop the onion.
Melt the margarine and gently fry it for 5 minutes, until soft but not brown.
Remove from the heat and stir in the breadcrumbs, sweetcorn, parsley, cheese, salt and pepper.
Pile the stuffing mixture on to the cod steaks.
Bake for 45 minutes, until the fish is cooked.
Serve with creamed potatoes and green beans.