Keller uses a roux, the traditional thickener made by cooking equal parts by weight butter and flour, for sauces and other dishes. For the smoothest sauces, add room-temperature or cold roux to a simmering liquid, or add cold liquid to a hot roux, to prevent the roux from seizing up.
Put the butter in a small skillet or saucepan and set it over medium heat. When it is almost melted, whisk in the flour and cook, whisking constantly and adjusting the heat as necessary so the roux bubbles but does not brown, 3 to 4 minutes. Transfer to a bowl or other container to cool, then store in the refrigerator for up to 2 weeks.
Recipe by Bistro OneSix at https://bistroonesix.com/ad-hoc-at-home-or-the-most-complex-soup-ive-ever-made/