White Onion Soup with Chive Oil

[kitchenelf]

For chive oil - bunch fresh chives, 1/2 cup extra-virgin olive oil

For soup - 2 pounds white onions (about 2 medium-large)
4 cups chicken broth (32 fluid ounces)
1/2 cup dry white wine
1 tablespoon fresh thyme leaves
1 tablespoon unsalted butter
1 teaspoon sugar
1/2 teaspoon white-wine vinegar
freshly ground white pepper to taste
a small wedge Parmigiano-Reggiano (about 4 ounces)

Make chive oil: 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.

Make soup: 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.

Makes about 6 cups.