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1
Preheat the oven to 400F.
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2
Put the butter in a small skillet and turn the heat to medium.
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3
Add the onion and a sprinkling of salt and pepper and cook, stirring occasionally, until the onion is soft but not yet beginning to brown.
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4
Turn off the heat.
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5
Transfer the onion to a flameproof ceramic or nonstick baking dish just large enough to contain the fish.
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6
Sprinkle the fish with salt and pepper and put it over the onion.
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7
Pour the cider around all and bring to a boil on top of the stove.
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8
Cover with foil and bake for 12 to 15 minutes, or until the fish is done (a thin-bladed knife inserted into its center will meet little resistance).
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9
Garnish with parsley and serve the fish with the onion and juices spooned over it.
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10
Clean 2 pounds mussels (page 208) and combine them with the cider in a covered pot.
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11
Turn the heat to high and cook, shaking the pan occasionally, until the mussels open, about 10 minutes.
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12
When they are cool enough to handle, remove them from their shells; pass the cider through a cheesecloth-lined strainer.
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13
Proceed as directed, using the mussel-scented cider for the liquid and adding the shelled mussels to the fish about halfway through its cooking time.
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14
In step 1, when the onion begins to soften, add 1 cup trimmed and chopped white, cremini, or portobello mushrooms and continue to cook, stirring occasionally, until the mushrooms have given up their liquid, about 10 minutes more.
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15
Proceed as directed.
