A Liaison Is The Richest Way To Thicken Your Sauces

You can transform a runny sauce into a lush sauce by incorporating a liaison. In culinary terms, a liaison can be cold butter and flour ("beurre manie"), roux, or a slurry of cornstarch or arrowroot that is added to a sauce as a thickening agent. Another frequently used liaison is a blended mixture of one-part egg yolk to three-parts heavy cream. When the liaison is slowly added to the hot sauce, the eggs coagulate and thicken the sauce, and together with the cream, add a rich velvety texture. A liaison improves the consistency of the sauce and also allows the flavors of the sauce to cling to your tongue and palate, thus increasing your enjoyment of what you're eating.

Making a liaison isn't difficult, but it can be a little tricky because improperly executed, the sauce could break down, and you'll wind up with scrambled eggs. Once you've beaten the egg yolk and heavy cream together, you'll need to temper the liaison before adding it to the sauce. Tempering is the process of gradually combining a hot ingredient with a cold ingredient to increase its temperature, which will prevent curdling. To temper the liaison, whisk together a little of the hot sauce into the beaten eggs and cream. Keep whisking as you add a little more hot sauce, then lower the heat to low, and slowly pour the liaison into the sauce, constantly stirring, until the sauce thickens.

The word "liaison" first appeared in English in the 1640s and means "union" or "binding together," which describes what it does for the broth-based sauces of white stews ("blanquettes"), like classic veal blanquette and chicken blanquette. Two rich sauces — Allemande and Normandy — are made by adding a liaison to a velouté, one of the five mother sauces of French cuisine. Velouté begins as a blond roux with chicken, veal, or fish stock whisked in until it binds together as a sauce, and with a few embellishments, it's often served as a gravy. 

Allemande sauce (also called "German sauce") starts off as a velouté that has been flavored with lemon juice, salt, pepper, and nutmeg, which is then enriched with the egg-cream liaison. Depending on the type of stock that is used, Allemande sauce can be served with chicken, veal, fish, eggs, or vegetables. Normandy sauce, which is most often a sauce for fish and seafood, starts as velouté that's made with fish stock. Chopped mushrooms are added, and then the sauce is thickened with the egg-cream liaison. Like Allemande sauce, Normandy sauce is very rich and would be especially delicious spooned over lobster tails.

A liaison isn't just for thickening savory sauces; it can also fix a runny chocolate mousse. The liaison is folded into the chocolate mixture, and when cooled and whipped, the mousse will have thickened to fluffy heights.