No one seems to know why soubise takes its name from Charles de Rohan (1715 —87), Prince of Soubise and Marshal of France, but there you go.
This thick sauce for meats and hard boiled eggs was originally made of chopped onions cooked in butter with cream. In later recipes, the cream is replaced with béchamel; in another version an onion purée is thickened with boiled rice, then pressed through a sieve before a big blob of butter is added.
Image by Hayford Peirce @ Citizendum