Which is one of the main ingredients of a roux?

Roux is a cooked mixture of fat (traditionally butter—clarified or whole—but could also be meat drippings or oil) and flour used to thicken sauces and soups. It’s usually a 1:1 ratio by weight of the two, cooked to varying degrees: white, blond, and brown. Cooking removes the raw starchy flavor of the flour and adds an underlying toastiness to the resulting sauce or dish.

It is the starting point of sauces such as velouté y béchamel. Velouté is made by making a white roux with butter and flour. Next, slowly stir in warm stock and let it simmer until a creamy, light sauce forms. A basic velouté can be used by itself on meats and vegetables or fashioned into numerous secondary sauces. Béchamel sauce is a sauce traditionally made from a white roux (butter and flour in a 1:1 mixture) and milk.