Düsseldorfer Senfrostbraten

The dish is a speciality of Düsseldorf and is built on the city's long mustard tradition. In 1726 the first mustard manufactory in Germany was founded on Düsseldorf's Ritterstraße — the firm later known by the ABB monogram of a subsequent owner, Adam Bernhard Bergrath — and the grinding method is said to have changed little since. Düsseldorf mustard (Düsseldorfer Mostert), sharp and pungent, became a regional emblem; in 2012 the names Düsseldorfer Mostert and Düsseldorfer Senf were registered as EU Protected Geographical Indications, reserving them for mustard made in the city. The Rostbraten itself is a regional brewhouse classic rather than a dish with a single documented inventor; what defines it is the local hot mustard baked onto the steak as a crust.

A thick rump steak, seared so the cut surface is brown and the inside still pink, under a crust that is the whole point of the dish. The topping bakes into a rough, golden layer — soft from onion, crisp from breadcrumb — and the Düsseldorf mustard hits sharp and hot, with a clean horseradish-like sting that cuts the richness of the beef and the butter it is seared in. The mustard heat is bracing rather than creamy, so each forkful is meat, sweet softened onion and a bright, pungent edge at once.

Searing the steak first runs the Maillard reaction on the cut surface, building the browned, meaty flavour, while a quick finish in a hot oven sets the topping without overcooking the inside. The crust works because the mustard, onion and breadcrumb mixture is thick and low in free water: the breadcrumbs absorb moisture and the surface dries and browns instead of steaming. Mustard's pungency comes from a glucosinolate that, when the seed is crushed and wetted, is converted by the enzyme myrosinase into allyl isothiocyanate, the volatile compound responsible for the sharp nasal heat; because that compound is heat-sensitive and volatile, baking is kept short so the crust sets but the mustard keeps its bite. Düsseldorf mustard is made hot and malty by milling brown and yellow mustard seed, which is why it drives the dish where a mild table mustard would not.