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.
It’s a thick sauce for meats, or hard boiled eggs, made of chopped onions cooked in butter with cream. In later recipes, the cream was replaced with béchamel; another version thickens an onion purée with boiled rice, which is then pressed through a sieve before added a big blob of butter.
Image by Hayford Peirce @ Citizendum