Sauce Robert
The science
One of the oldest named French sauces (it predates Carême's codification — Rabelais mentions it in the 16th century). The onion is cooked soft and sweet; the white wine adds acidity; and the mustard, stirred in off the heat to preserve its volatile pungency (the isothiocyanate chemistry described under Sauce Moutarde), gives the finishing sharpness that defines it. It is the brown-sauce world's answer to mustard sauce.
Flavor profile
Onion sweated in butter, deglazed with white wine, simmered with demi-glace, and finished with mustard off the heat.
Culinary uses
Grilled and pan-fried pork, sausages, and rich meats whose fat the mustard cuts.
Reference notes
Parent: Demi-glace. Cross-family echo: Sauce Moutarde (béchamel). Direct child: Charcutière. Historical note: among the oldest surviving named sauces.