1. Heat the oven to 450°. Put the potatoes in a medium saucepan of salted water. Bring to a boil and continue boiling until tender, about 15 minutes.
2. 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.
3. 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.
4. 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.
Wine Recommendation: This comforting dish will find its ideal wine soul mate in just about any chardonnay. Try a bottle from California or a more refined version--a Rully, Montagny, or Mâcon-Villages,for example--from southern Burgundy in France.