In the culinary arts, liaison is word that broadly describes the process of thickening a sauce using starch (such as flour or cornstarch), egg yolks, fat, even foie gras or puréed vegetables.
Most commonly, however, the word liaison refers to a mixture of egg yolks and heavy cream that is used to thicken a sauce.
You might sometimes see it called liaison finale, or final liaison, because it is (both by tradition and necessity) the last thing that goes on the plate.
By necessity because in most cases a liaison will break or curdle if it is boiled (although a few moments of gentle simmering are usually fine).
A very simple liaison is achieved using the technique known as monter au beurre (famously described by Anthony Bourdain in his book Kitchen Confidential), which involves, basically, stirring little chunks of butter into your sauce right before you serve it. This works a little bit like the way beurre blanc works — which is to say that the fat droplets bind with the liquid in the sauce to form an emulsion — thickening and enriching the sauce.
Technically, adding heavy cream to a sauce is a form of liaison, to the extent that the fat in the cream behaves like the butter in monter au beurre. You need to use heavy cream, though. Half and half or milk will curdle.
Some chefs produce a liaison using egg yolks combined with blood (not their own) or, even more exotically, lobster roe.
A cornstarch slurry, which is a particularly useful sauce thickener, is also a form of liaison, as is beurre manie, in which raw flour and butter are combined and then whisked into a simmering sauce. (Note that beurre manie differs from roux, a blend of flour and butter which is cooked before whisking hot liquid into it.)
- In a stainless steel or glass bowl, beat together the cream and egg yolks until smooth. This egg-cream mixture is your liaison.
- Slowly add about a cup of the hot velouté into the liaison, whisking constantly so that the egg yolks don't curdle from the heat.
- Now gradually whisk the warm liaison back into the velouté.
- Bring the sauce back to a gentle simmer for just a moment, but don't let it boil, or it will curdle.
Also see: How to Thicken a Sauce with Roux