Years later I learned that the French name for the sauce, before the butter addition, was sauce espagnole.
From the washingtonpost.com
And just like a sauce espagnole can enhance many different dishes, Tse's dipping sauce is equally versatile.
From the sfgate.com
Make the sauce espagnole first.
From the courier-journal.com
Sauce espagnole in a restaurant kitchen is made by the apprentice sous chefs who simmer veal and beef bones with vegetables for hours to reduce to a rich brown sauce.
From the courier-journal.com
Espagnole sauce can be reduced to a silky viscosity called demi-glace, which can further be used in recipes to add depth of flavor.