Welsh du Nord

The dish descends from British Welsh rabbit, a jocular name for melted cheese on toast first recorded in 1725, later softened to Welsh rarebit, a corruption of rabbit attested from 1781 and popularised by Francis Grose's dictionary in 1785 — there is no rabbit in it. How it crossed to northern France is not documented: the popular stories range from Welsh soldiers at the sieges of Boulogne in 1544 to 18th- and 19th-century British miners and the British sailors, fishermen and tourists of the Opal Coast, but no confirmed source supports them. What is clear is that it took root in the estaminets of the Nord, is said to have spread from around Boulogne-sur-Mer along the Côte d'Opale, and is now a fixture of brasseries across the region and in Paris.

Under a blistered, gratinéed crust the sauce is thick, stretchy and savoury, smelling of toasted cheese, malt and the sharp bite of mustard. The toast underneath has soaked up enough of it to soften but should hold a little structure rather than turn to mush, and the ham adds salt and chew. The beer cuts through the richness with a faint bitterness and acidity; the mustard keeps it from being merely heavy. With the complet, the egg yolk breaks and runs into the cheese, and hot, crisp frites are dragged through the lot.

Melting cheese into beer rather than just grilling it gives a pourable sauce: the warm liquid loosens the cheese, and the beer's water and mild acidity help the curd flow without the proteins seizing and weeping fat. Mustard and beer work mainly as flavour and a touch of acidity that lift the richness, not as the thing that smooths the melt — that comes from gentle heat and the cheese's own proteins. Once poured over the toast, a spell under the grill drives the Maillard reaction at the surface: cheese proteins and sugars brown, building the toasted, faintly nutty crust and the deeper savoury notes that distinguish a baked Welsh from plain melted cheese.