1. Heat the oven to 450°. In a large roasting pan, toss the potatoes with 3 tablespoons of the oil, 1/2 teaspoon of the salt, and 1/4 teaspoon of the pepper. Scatter the thyme sprigs or 1 1/2 teaspoons of the dried thyme on top. Roast the potatoes for 10 minutes. Stir and continue cooking for 10 minutes longer.
2. Meanwhile, coat the fish with the remaining 2 tablespoons oil and sprinkle the remaining 1/2 teaspoon salt, 1/4 teaspoon pepper, and 1/2 teaspoon dried thyme or 1 1/2 teaspoons of the chopped fresh thyme on the fish.
3. Remove the pan from the oven. Stir the potatoes and push them to one side of the pan. Put the fish on the other side of the pan and return the pan to the oven. Roast the fish and potatoes until just done, about 10 minutes longer.
4. Remove the fish. Toss the potatoes with the remaining 1 teaspoon chopped fresh thyme, if using. Serve the fish with the potatoes alongside.
Variations: You can use other firm white fish fillets, such as scrod, halibut, or haddock, in place of the cod in this recipe.
Wine Recommendation: These earthy flavors beg for a vibrant white wine to provide contrast. Any white based on the sauvignon blanc grape--especially an unoaked type, such as a French Sancerre or Pouilly-Fumé, a sauvignon from northeast Italy, or a New Zealand sauvignon blanc--will be terrific.