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1
In a medium saucepan, melt the 3 tablespoons of butter.
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2
Add the onion and cook over moderate heat, stirring occasionally, until softened, about 5 minutes.
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3
Add the beer, cover and cook over low heat until the onion is very soft, about 30 minutes.
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4
Drain the onion in a sieve set over a heatproof bowl; reserve the cooking liquid.
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5
Transfer the onion to a blender and puree until smooth.
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6
Scrape the onion puree into a small saucepan and season with salt and white pepper.
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7
Preheat the oven to 400.
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8
Return the onion cooking liquid to the saucepan.
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9
Boil over high heat until reduced to 1/4 cup, about 3 minutes.
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10
Remove from the heat and add the lemon juice.
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11
Swirl in the 2 tablespoons of cold butter, 1 tablespoon at a time.
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12
Season with salt and white pepper.
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13
Oil a 9-by-13-inch baking dish.
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14
Arrange the cod in the dish and drizzle with olive oil; rub the oil all over the fish.
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15
Season the cod with salt, white pepper and piment d'Espelette and sprinkle with the thyme.
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16
Bake for about 15 minutes, basting a few times with the pan juices, until the cod is just opaque throughout.
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17
Gently reheat the onion puree and onion sauce; do not let the sauce boil.
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18
Spoon the onion puree onto plates.
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19
Set the cod on the puree and spoon the onion sauce on top.
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20
Serve right away.
