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1
Preheat the oven to 450.
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2
In a large skillet, melt 1 tablespoon of the butter.
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3
Add the leeks and cook over moderately high heat, stirring, until softened, 2 to 3 minutes.
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4
Add the mushrooms, season with salt and pepper and cook, stirring, until the vegetables are tender and the juices have evaporated, about 5 minutes.
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5
Stir in 1 tablespoon of the parsley and remove from the heat.
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6
Coat a 13-by-9-inch baking pan with 1 tablespoon butter.
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7
Layer one-third of the potatoes in the pan, overlapping slightly; sprinkle with salt and pepper.
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8
Spoon half of the leek mixture over the potatoes, top with another layer of potatoes and season with salt and pepper.
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9
Repeat with the remaining leeks and potatoes.
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10
Sprinkle with salt and pepper, dot with the remaining 1 1/2 tablespoons butter and pour the stock around the edges.
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11
Bake in the upper third of the oven for 25 minutes, or until the potatoes are tender.
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12
If they start to overbrown, cover loosely with foil.
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13
Rub the cod with the oil, season with salt and pepper and set it on the potatoes.
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14
Scatter the tomato and the remaining 1 tablespoon parsley over the fish.
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15
Bake uncovered for 8 to 10 minutes, or until the cod is just cooked through.
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16
Serve immediately , spooning any pan juices over the potatoes.
