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1
Preheat the oven to 500 degrees; if you have a baking stone, use it.
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2
Finely grate the zest of the tangerines, without grating any of the bitter white part.
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3
(On a good grater, you should be able to get away with one swipe per section of skin, rather than rubbing over and over.)
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4
Set aside.
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5
Juice the tangerines, and measure 1 1/4 cups of the juice.
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6
Set the juice in a small saucepan over medium-high heat, bring to a boil and reduce, stirring occasionally, to about 1/2 cup; this will take at least 10 minutes.
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7
Place a large ovenproof skillet, preferably nonstick, over high heat, and add the tablespoon of olive oil.
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8
If necessary, cut the cod into serving pieces so it will fit into the skillet.
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9
When the oil smokes, pour out any excess.
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10
(You need only a film.)
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11
Add the fish and put the skillet in the oven; set a timer for 6 minutes.
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12
The fish is done when it offers little or no resistance when a thin-bladed knife passes through it and when the flesh is opaque, or nearly so, throughout.
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13
Unless unusually thick, it will be done in about 8 minutes.
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14
While the fish is cooking, season the tangerine sauce with salt and cayenne; it should have something of a bite.
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15
Add the coriander, if you choose.
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16
Whisk in the butter or oil a little at a time, until the sauce is smooth and thick.
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17
Taste, adjust seasoning and add the grated zest; cook 15 seconds longer.
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18
Serve the fish with the sauce spooned over it, and garnish with the herb.
