Sauté onion, celery, carrots, and garlic in oil in a 6-quart heavy pot over moderately high heat, stirring, until golden, about 10 minutes. Add wines and simmer, stirring occasionally, until mixture is reduced to 1 quart, about 1 hour.
Pour through a sieve into a saucepan and simmer until reduced to 2 cups. Reheat reduction and season with salt and pepper before using.
Reduction may be made 2 days ahead, cooled completely, and chilled, covered. Reheat before using.