You can substitute bass, hake, or haddock for the cod in this one-pot casserole.
Preheat oven to 425 degrees. Using amandoline, slice potatoes a scant1/8 inch thick. Drizzle 1 tablespoonoil in a shallow 3-quart lidded casserole. In slightly overlappingcircles, alternate one-third eachof potatoes, tomatoes, and shallots(about 1 tomato and shallot slicefor every 5 or 6 potato slices).Drizzle with 1 tablespoon oil, scatterwith one-third of thyme leaves,and season with salt and pepper.Repeat twice.
Roast, covered, until potatoesare knife-tender, 50 minutes. Seasonfish with salt and pepper; placelemon slices over potatoes, and fishon top. Drizzle with remaining oil;add thyme sprigs. Cover; roastuntil fish is cooked, 12 to 15 minutes.Let cool slightly before serving.