The classic method of thickening sauces is to add roux - and sometimes cream as well. Another very easy method is to use a mixture of butter and flour called beurre manié to thicken sauces. To make the mixture, equal quantities of soft butter and flour are kneaded into a dough, cut into small cubes and then frozen. When refrigerated, beurre manié stays fresh for up to fourteen days. To thicken sauces, a cube of beurre manié is stirred into the sauce and then brought to the boil with the sauce. The advantage of beurre manié compared with roux is that the sauce can be brought to the boil again. This isn't possible with roux because lumps would form in the sauce.