The sauce for the Rouelles de Veau Bourgeoise is thickened and enriched with egg yolks just before serving. This final, lightly cooked thickening is known as a liaison (the meaning is the same as in English), and may also be achieved with cornflour, arrowroot, or a flour and butter paste known as a beurre manié, all of which require only a brief cooking. If you are using an egg yolk liaison, temper (partially cook) the yolks first with a little of the hot cooking liquid to prevent them becoming grainy. A flourless sauce bound wirt egg yolks must never be boiled.