A liaison is all you need if you want to turn a runny sauce into a lush one, and one of the most used liaisons is a blend of one-part egg yolk to three-parts heavy cream.
It improves the consistency of the sauce and allows the flavors of the sauce to cling to your tongue and palate. To make one, first beat the egg yolk and heavy cream together.
Next, temper the liaison before adding it to the sauce to prevent curdling. To do this, 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 and slowly pour the liaison into the sauce, constantly stirring, until the sauce thickens.