*Percent Daily Values are based on a 2,000 calorie diet. Your daily values may be higher or lower depending on your calorie needs.
**Nutrient information is not available for all ingredients. Amount is based on available nutrient data.
(-)Information is not currently available for this nutrient. If you are following a medically restrictive diet, please consult your doctor or registered dietitian before preparing this recipe for personal consumption.
Bring onion and water to a boil in a saucepan. Cover, reduce heat, and simmer until tender, 15 to 20 minutes. Transfer onion and water to a blender; puree until smooth.
Heat oil in a saucepan over medium heat. Whisk in flour to form a thick paste the consistency of cake frosting. Continue cooking, whisking constantly, until flour has turned golden and begins to smell like toast, about 20 minutes.
Stir onion puree into flour paste. Cook and stir until thickened to sauce consistency, adding more water if necessary, about 2 minutes. Stir in wine vinegar and salt.
Onion can also be cooked in water in microwave on high until tender, about 10 minutes.
Do not omit or substitute wine vinegar with any other type of vinegar. Cooked onions taste somewhat sweet, and wine vinegar gives the sauce just the right taste balance. Alcohol, cider or balsamic vinegar are not a good option because they would completely change the distinctive taste of the sauce.