How to Make It
Heat the oven to 450&176;. Put the potatoes in a medium saucepan of salted water. Bring to a boil and continue boiling until tender, about 15 minutes.
Drain the potatoes and put them back into the saucepan along with 3/4 teaspoon of the salt and 1/4 teaspoon of the pepper. Mash the potatoes over very low heat, gradually incorporating the butter and cream.
Rub the cod with the oil and sprinkle with the remaining 1/2 teaspoon salt and 1/4 teaspoon pepper. Put the cod in a large roasting pan, sprinkle the fillets with the garlic, and then spread with the mashed potatoes. Roast until the fish is nearly done, 8 to 10 minutes for 3/4-inch-thick fillets.
Heat the broiler. Broil the fish until the mashed potatoes have a golden brown crust, about 2 minutes.
Fish Alternatives Use other relatively thick, white-fleshed fillets in place of the cod. Good choices include grouper, haddock, orange roughy, red snapper, and turbot.