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