Tarte a l'Oignon

Tarte a l'oignon is a savory tart of Alsace, where the Alsatian dialect calls it Zewelwai — a cousin of the German Zwiebelkuchen of the Upper Rhine valley. The tart belongs to the autumn season in Alsace, traditionally eaten with the Federweisser — the lightly fermented new wine — that arrives at the same time. The base is commonly a yeast dough or a short pastry, distinguishing it from the puff-pastry quiche of neighboring Lorraine; the filling is onions slowly cooked until deeply softened and sweet, bound in a cream-and-egg custard. The tart is eaten warm, cut into wedges, often as a light meal with a glass of the new wine.

A wedge of tarte a l'oignon brings up a soft, slightly bready base beneath a thick layer of sweet, jammy onions in a glossy cream-and-egg custard. The flavor is dominated by the slow-cooked onions — sweet, deep, almost mellow — lifted by the richness of the cream and the faint yeasty note of the base. Eaten warm with a glass of new wine, it is a dish of the autumn harvest.

Tarte a l'oignon rests on two mechanisms: caramelized onions and a baked custard. The onions are cooked very slowly in butter until their cell walls break down, their moisture evaporates, and their natural sugars concentrate and brown through Maillard reaction into a sweet, jammy mass. The custard — beaten eggs and cream — is poured over the cooked onions in their pastry shell; during baking, the egg-yolk proteins coagulate to set the custard, binding the onions into a sliceable layer. The yeast-dough base, where used, gives a softer, breadier platform than shortcrust would, distinguishing the Alsatian tart from the quiche; the base is partially baked blind before the filling goes in, to prevent it from going soggy beneath the wet custard.