What is "soubise sauce"

Soubise sauce is an onion sauce. It is based on Béchamel sauce, with the addition of onion purée.

Soubise sauce was originally a slow-cooked onion and rice mixture used on roasted meats, game, poultry and vegetables. Over the years, and through the hands of many chefs, it now has many variations. The simplest contains 3 ingredients, not counting salt and pepper. Most current chefs omit the rice and the cream in favor of onion, butter and beef broth. The finished product is very creamy and rich when pureed.