Soubise is a French culinary term that refers to a type of onion sauce. It is made by cooking onions slowly until they become soft and sweet, then combining them with a béchamel sauce, which is a creamy white sauce made from butter, flour, and milk. This sauce is often used to enhance the flavor of various dishes, particularly vegetables and meats. The name "Soubise" is derived from the Duke of Soubise, a French nobleman in the 18th century. The sauce is commonly served with dishes like gratin or as a topping for fish and poultry, adding a rich and savory element to the meal.