Sauce Soubise
The science
Long, gentle cooking of the onions converts their sharp, sulfurous raw compounds and breaks down cell walls into a smooth, sweet purée without Maillard browning (which would make it taste of caramelized onion rather than the pale, delicate Soubise intended). The béchamel carries this purée as a suspension; some versions thicken with rice cooked in the onions instead of, or alongside, roux.
Flavor profile
A purée of onions — slowly sweated or "smothered" in butter until meltingly soft and sweet, never browned — folded into béchamel; often enriched with cream.
Culinary uses
With white meats, eggs, lamb, and as a bed for poached or roasted poultry.
Reference notes
Parent: Béchamel. Technique: sweating / smothering aromatics. Compare to the rice-thickened Soubise variant as a bridge to non-roux thickening.