No matter who does it—even Julia Child--occasionally a sauce will thin during cooking. Instead of correcting it with more flour, add a little arrow root to a little water and stir it in. Arrow root thickens without any cooking, thus you can get exactly the consistency you want. (If you do a lot of cooking with sauces, you should know that buying arrow root from Penzey’s is much less expensive. You can buy a couple of cups in a foil pack for about the same price as what you get in that funny little bottle of McCormick’s at the grocery.)
If correcting an egg yolk based sauce you can often—but not always—do so by vigorously whisking in another yolk. Usually when you have this problem it is because you held it too long after completion. It is usually best to do egg yolk sauces just as you plan to use them.
