“Au Beurre” is a French expression that means “with butter.”
It can mean that a food item has been cooked or glazed in butter, or some butter added as a finishing touch.
Sometimes the type of butter used might be specified: for instance, “au beurre salé” means “with salted butter.”
Monter au beurre
When butter is added as a finishing touch to sauces, it’s called “monter au beurre.” To do this, chunks of cold butter, usually unsalted, are whisked in until they melt. The added fat gives the sauce better “mouth feel”, making it taste richer.