The sauce you need is based on a foamy mixture called a sabayon (a cousin of the light, egg-based Italian dessert zabaglione). A sabayon is made by beating egg yolks with a liquid over simmering water until thickened and increased in volume. ( the liquid can be water, but champagne or wine is often used for a savoury sabayon.) The sabayon must not get too hot during cooking or it will become grainy: if it begins to feel warmer than body temperature, remove the pan briefly from the heat, beating continuously, until the mixture cools. Then return the pan to the heat and continue cooking. Sabayon may be served warm or cold; a cold sabayon is beaten off the heat until cooled.
Sabayons may be sweet or savoury. They may be served simply as a sauce, often flavoured with an alcohol, or they may form the base of some mousse mixtures. They may also be used for sweet or savoury gratins. Cornflour is sometimes added for stability.