This modernist Sauce Soubise (see Chapter 6, “White Sauces,” for the other two versions) is based on a fluid gel made from “milk” extracted from onions. Agar is the emulsifier. This recipe is adapted from one invented by Heston Blumenthal.
In a wide, heavy, straight-sided sauté pan, sweat the onions in the butter until the onions are completely soft but not at all browned. Pour over the milk and maintain at a very low simmer for 20 minutes or until the milk has reduced by about half.
Strain the onions through a chinois, pressing down on them, and measure out 1