White Onion Soup with Chive Oil Recipe

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