From NY Times, a really nice simple recipe..
Ingredients
4 or 5 medium potatoes
6 tablespoons melted butter
salt and pepper
1-1/2 lbs Cod skinned in 2 or more pieces
Instructions
1. Heat oven to 400 degrees. Peel potatoes, and cut them into slices about 1/8-inch thick (a mandoline comes in handy here). Toss the potatoes in an 8-by-12-inch or similar size baking pan with 4 tablespoons of the butter. Season the potatoes liberally, spread them evenly and place the pan in the oven.
2. Cook for about 40 minutes, checking once or twice, until the potatoes are tender when pierced with a thin-bladed knife and have begun to brown on top. Remove the potatoes from the oven. Turn on the broiler, and adjust the rack so that it is 4 to 6 inches from the heat source.
3. Top the potatoes with the fish, drizzle with the remaining butter and sprinkle with some more salt and pepper. Broil until the fish is done, 6 to 10 minutes, depending on its thickness (a thin-bladed knife will pass through it easily). If at any point the top of the potatoes begins to burn, move the pan a couple of inches farther away from the heat source.