CREAMY BAKED SALT COD

Steps:

• To remove the salt from the salt cod, put it in a large, deep container that fits in your sink, under the faucet. Run fresh cold water over the fish, filling the container completely so water is spilling over the brim. Keep a slow, steady drip of fresh water going as you soak the baccalà for at least 12 hours, up to a couple of days. Occasionally drain the water and refresh (which you can do whenever you need the sink). If you can't keep the drip going, change the water every 4 hours. To decide when baccalà has been soaked sufficiently, lift the fish out of the water, press a finger against it at a thick part, and touch it to the tongue. If it's palatably salty, it is ready to be cooked. Drain the baccalà, pat dry, and cook within a day or so; meanwhile, store it in the refrigerator, well wrapped.
• Cut the baccalà into chunks about 3 inches square, and dredge on all sides in flour. Put the butter and olive oil in the saucepan, and set over medium heat. When the butter is foaming, lay in a batch of floured codfish chunks in one layer, not crowded. Cook and color the underside for several minutes, turn the chunks, and crisp and color the other sides. When lightly browned all over, remove to a platter. Fry the remaining chunks the same way.
• Dump the onion slices into the pan, and toss well in the remaining fat. As the onions start to sizzle and wilt, pour in 1/2 cup water, and stir and scrape up the browned bits on the pan bottom. Cook the onions, stirring frequently, until wilted and translucent, 8 to 10 minutes. Meanwhile, heat the milk in a separate pan, almost to a simmer.
• Lay the baccalà chunks on top of the caramelized onions, nestling all the chunks in one layer. Pour in any juices from the fish platter, and enough hot milk to cover the fish completely. Over medium heat, bring the milk to a gentle bubbling simmer. Set the cover slightly ajar, and simmer the baccalà for about an hour and a half, until the fish chunks break apart when stirred.
• Uncover the pan, and maintain the slow simmer to gradually reduce the milk (which by now has curdled and separated-no cause for concern). Stir the fish occasionally, and break up the flakes into smaller pieces. As the moisture evaporates, lower the heat and scrape up any crust on the pan bottom, to avoid burning.
• Cook uncovered, slowly, for 2 to 2 1/2 hours, until the baccalà and milk have melded into a thick sauce, about half the original volume. (If you need to shorten the cooking time, boil the milk over higher heat, but be sure to stir the baccalà and scrape the saucepan frequently.)
• When it is fully cooked and thickened in the saucepan, pour the hot baccalà into the baking dish and spread it in an even layer. Heat the oven to 350˚, and set a rack in the center. Sprinkle the grated Grana Padano or Parmigiano-Reggiano over the top of the baccalà, and bake for about an hour, or until the top is crusty and browned.
• Bake slices of firm polenta, topped with grated cheese, during the last 15 or 20 minutes that the baccalà is in the oven. To serve, put a slice or more of crisp polenta in a plate, and spoon some hot baccalà alongside.