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1
Cut leeks crosswise into 2-inch lengths, then cut lengthwise into enough very thin strips to measure 2 cups.
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2
Wash leek strips in a bowl of cold water, agitating them, then lift out and pat dry.
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3
Heat oil in a deep 4-quart heavy saucepan until it registers 360F on thermometer.
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4
Fry leeks in 8 batches, stirring, until golden, about 10 seconds per batch.
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5
Transfer as fried with a slotted spoon to paper towels to drain.
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6
Cool completely (leeks will crisp as they cool).
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7
Pick over oysters, discarding any bits of shell, and rinse well.
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8
Wash chopped leeks in a bowl of cold water, agitating them, then lift out and drain well.
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9
Peel potatoes and cut into 1/2-inch cubes.
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10
Cook leeks, potatoes, and salt in butter in a 4-quart heavy saucepan, covered, over low heat, stirring occasionally, until leeks are golden and potatoes are beginning to soften, about 15 minutes.
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11
Add water and simmer, covered, over moderate heat until potatoes are very tender, about 10 minutes.
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12
Puree soup in batches in a blender until very smooth (use caution when blending hot liquids), transferring to a bowl.
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13
Return soup to saucepan.
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14
Add oyster liquor and half-and-half and bring to a simmer over moderate heat, stirring occasionally (do not boil).
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15
Add oysters and cayenne and cook, stirring occasionally, just until oysters become plump and edges curl, about 3 minutes.
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16
Season with salt.
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17
Serve soup topped with fried leeks.
