Cook spinach according to package directions. Let stand until cool enough to handle. Drain spinach in colander; squeeze out as much liquid as possible. Transfer spinach to a cutting board, and chop with a large knife.
Melt butter in a medium saucepan over medium heat. Add shallot; cook 2 minutes. Add cream, and bring to a boil. Stir in spinach, 1/2 teaspoon salt, 1/4 teaspoon pepper, and nutmeg. Cook 5 minutes, stirring constantly, until mixture has thickened.
Season cod with remaining 1/2 teaspoon each of salt and pepper, and place in a lightly buttered 2 1/2-quart baking dish. Spread creamed spinach over fillet to cover. Combine breadcrumbs and cheese until well blended; sprinkle over top of fillet.
Bake at 400° for 28 to 30 minutes, or until fish is just opaque in thickest part. Cut into 6 portions, and transfer to plates, spooning any creamed spinach and juices from sides of dish over portions. Serve immediately.
Casserole Basics: Glass and ceramic (earthenware) dishes work best for casserole cooking because they heat up quickly and evenly. If you substitute a metal baking pan, the casserole usually cooks more slowly, so you may have to increase the oven temperature by 25° to compensate.
Always start with the freshest seafood available and pat it dry. If you use individual steaks or fillets, select pieces that are at least 1 inch thick. If a single fillet from a large fish tapers to a thin end, fold the thin end under to help the fillet cook uniformly.
If the fillet in the casserole is thick, let it sit at room temperature while you prepare the other ingredients for the dish. It will cook more quickly.
Thick fillets or steaks, especially if covered with a sauce or topping, can take from 12 to 15 minutes of baking time per inch. To check for doneness, cut into the fillet at the thickest part to see if it's opaque inside and pulls apart with little resistance.
Casserole recipes calling for fully or partially cooked seafood or fish need only enough time in the oven to get hot, so an instant-read thermometer comes in handy. If the middle of the casserole registers 160°, it's done.
If you're making a casserole with cooked seafood that's added to a sauce, heat the sauce separately, stir in cold, cooked fish, and then assemble the dish. This will speed up the time in the oven and keep the fish from disintegrating.
Don't cook fish until it flakes easily-- by that point, the fish has given up all of its juices, rendering it dry and leaving a lot of water in the dish. If this occurs, spoon the juices back over the fish when serving.
Remember, the casserole will continue cooking once it's pulled from the oven, so it's better to take it out sooner rather than later.