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 butter mixture used with various meats, game, poultry and vegetables dating back to the 1850s. 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. Some current chefs favor altering the sauce with beef stock along with the traditional onion, butter and cream.