Put one tablespoonful of butter into a saucepan. When it bubbles add one tablespoonful of flour and let them cook together for a few minutes, stirring all the time. If it is to be used as thickening for a white sauce or soup, do not let it color. If for brown soup or sauce, let it become brown. This amount is sufficient to thicken one cupful of milk or of stock, to make a sauce, or to thicken one pint or more of soup.
Roux can be prepared and kept in jars ready for use. The proportion of equal quantities of butter and flour is usually taken, and is the rule, but in some cases double the flour is used. The flour cooked in this way gives a better result than when rubbed with the butter and stirred into the liquid. Cooking flour in hot fat seems to more surely burst the starch-grains, which removes the raw taste it is likely to have if cooked only in the boiling liquid.