From Marcella Hazan's Essentials of Classic Italian Cuisine
1½ pounds boiling potatoes (such as Yukon Gold)
A bake and serve dish, approximately 10 by 16 inches, preferably enameled cast iron ware
½ cup extra virgin olive oil
1 Tablespoon chopped garlic
¼ cup chopped parsley
Freshly ground black pepper
2 halibut fillets, approximately 1 pound each, or the equivalent of other thick, firm fish fillets (such as bluefish, sea bass or sable)
Preheat the oven to 450 degrees.
Peel the potatoes and slice them very thin, barely thicker than chips. Wash them in cold water, then pat them thoroughly dry with a cloth kitchen towel.
Put all the potatoes into the baking dish, half of the olive oil, half the garlic, half the parsley, and several liberal pinches of salt and black pepper. Toss the potatoes 2 or 3 times to coat them well, then spread them evenly over the bottom of the dish.
When the oven reaches the preset temperature, put the potatoes in the uppermost third of the oven and roast them for 12 to 15 minutes, until they are about halfway done.
Take out the dish, but do not turn off the oven. Put the fish fillets over the potatoes (skin side down if you have left the skin on). Mix the remaining olive oil, garlic and parsley in a small bowl and pour the mixture over the fish, distributing it evenly. Sprinkle with liberal amounts of salt and pepper. Return the dish to the oven.
After 10 minutes, take the dish out, but do not turn off the oven. Use a spoon to scoop up some of the oil at the bottom of the dish and baste the fish with it. Loosen those potatoes that have become browned and are stuck to the sides of the dish, moving them away. Push into their place slices that are not so brown. Return the dish to the oven and bake for 5 to 8 minutes more depending on the thickness of the fish fillets.
Remove the dish from the oven and allow to settle for a few minutes. Serve directly from the baking dish, scraping loose all the potatoes stuck to the sides – they are the most delectable bits – and pouring the cooking juices over each portion of fish and potatoes.