A _roux_ is a mixture of butter and flour _cooked_. It is generally
added, uncooked, to thicken a sauce or a soup; but the flavor is much
better if it is first cooked, and the sauce or soup is added to _it_.
Professional French cooks always manage it in this way. When the butter
is first brought to the boiling-point, in a small stew-pan or cup, the
sifted flour is sprinkled in, and both are mixed well together over the
fire with an egg-whisk, until the flour is well cooked; a part of the
sauce or soup is then stirred in until it becomes smooth and thin enough
to add to the main sauce or soup. If the _roux_ is intended for a white
sauce, it is not allowed to color; if for a brown sauce, it may color a
little, or browned flour may be used.