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1
Simmer raisins in vinegar in a small saucepan until raisins are plump, 3 to 5 minutes, then cool.
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2
Cook onion in 1 tablespoon oil in a 12-inch nonstick skillet over moderate heat, stirring occasionally, until softened, 4 to 6 minutes.
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3
Remove skillet from heat and transfer half of onion to a 9-inch square glass or ceramic baking dish with a slotted spoon, spreading evenly.
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4
Reserve remaining onion.
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5
Cut fish into 6 equal pieces.
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6
Pat dry and season with salt and pepper.
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7
Spread potato starch on a plate and dredge fish in it, shaking off excess.
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8
Heat remaining 1 1/2 tablespoons oil in skillet over moderately high heat until hot but not smoking, then saute fish, skin sides down, until lightly browned, 1 1/2 to 3 minutes.
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9
Turn fish over and saute until lightly browned and just cooked through, 1 1/2 to 3 minutes more.
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10
Arrange fish on top of onion in baking dish, then top with reserved onion and drizzle with raisins and vinegar.
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11
Cool completely, then marinate, covered and chilled, at least 8 hours.
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12
Bring to room temperature.
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13
Tear enough fennel fronds to measure 1 tablespoon.
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14
Trim stalks flush with bulb and discard stalks.
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15
Quarter bulb lengthwise, then cut lengthwise into paper-thin slices with a mandoline or other manual slicer.
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16
Transfer slices to a bowl and toss with radish sprouts.
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17
Whisk together vinegar and oil, then whisk in sugar and salt and pepper to taste.
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18
Toss vegetables with half of dressing and divide among 6 plates.
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19
Remove onion from top of fish, reserving it, and put a piece of fish on top of each salad.
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20
Mound all of onion and raisins on top of fish, then drizzle with remaining dressing and sprinkle with fennel fronds.
