1. Preheat the oven to 200°C.
2. Wash the leeks and cut into 1 cm pieces. Boil the leeks in a pressure cooker with the lid closed for 5 minutes with half a chicken stock cube.
3.  While the leeks are cooking, peel and cut the onions. Brown the onions in a pan with a little butter.
4. Drain the leeks then season to taste. Place them in a buttered gratin baking dish. Place the four cod fillets in the dish.
5. Season and sprinkle the onions over the top.
6. Place in the oven for around 30 minutes.

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