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1
Have ready a bowl of ice and cold water.
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2
In a saucepan of boiling water blanch chives 10 seconds and drain in a sieve.
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3
Immediately plunge chives into ice water to stop cooking.
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4
Remove chives from ice water, squeezing out excess water, and drain on paper towels.
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5
Chop chives and in a blender puree with oil until smooth.
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6
Transfer chive oil to a bowl.
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7
Chill oil, covered, at least 8 hours and up to 1 day.
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8
Pour oil through a fine sieve into a small bowl, pressing hard on solids.
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9
Thinly slice onions and in a 5-quart heavy kettle combine with broth and wine.
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10
Bring mixture to a boil and skim foam.
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11
Add thyme and simmer, uncovered, until onions are very tender, 45 to 50 minutes.
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12
Cool mixture 10 minutes.
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13
In a blender puree mixture in batches with butter until smooth (use caution when blending hot liquids), transferring to a bowl.
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14
In kettle stir together puree, sugar, vinegar, pepper, and salt to taste and heat over moderate heat, stirring, until heated through.
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15
Soup may be made 2 days ahead and cooled, uncovered, before chilling, covered.
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16
Reheat soup before serving.
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17
With a vegetable peeler shave enough thin slices from Parmigiano-Reggiano to measure about 1/2 cup.
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18
Serve soup drizzled with chive oil and topped with Parmigiano-Reggiano.
