This dish is excellent as is; however you can add a teaspoon or so of minced garlic; and/or some fresh or dried herbs; a tablespoon of so of curry powder, chili powder, paprika; or a pinch of cayenne or red pepper flakes to the potatoes before roasting.
2 lbs potatoes
6 T melted butter or extra-virgin olive oil, divided
salt and pepper, to taste
1 1/2 lbs cod or other white-fleshed fish fillets, in 2 or more pieces
1. Preheat the oven to 400ºF. Peel the potatoes, if you like, and cut them into slices about 1/8-inch thick (or as thin as you can). A mandoline is good for this task. Toss the potatoes in an 8- x -11-inch or similar-sized baking pan with 4 T of the butter or oil. Season the potatoes liberally with salt and pepper, spread them evenly in the baking dish, and put the pan in the oven.
2. Cook for about 35 minutes, checking once or twice, until the potatoes are tender when pierced and have begun to brown on top. (This will depend on the thickness of the slices.) Remove from oven and place pieces of fish on top of potatoes. Drizzle the fish with the remaining 2 T butter or oil and return to the oven. Bake until the fish is tender, from 5 to 10 minutes (this, too, will depend on the thickness of the fillets). You can also do this last step under the broiler, if you like–the fish cook more quickly. Just make sure that the top of the potatoes doesn’t start to burn; if they do, move the pan a couple of inches farther away from the heat source.