The tarte à l’oignon alsacienne (Alsatian onion tart, Zewelkueche in dialect) is a savoury open tart of slowly caramelised onions in a cream and egg custard on a bread dough or shortcrust base — the dish that fills every winstub on Tuesday evenings (traditionally, Zewelkueche day across Alsace) and appears at every wine festival alongside a glass of new-vintage Sylvaner. The onions are the heart: 1kg of yellow onions (for 4 portions), sliced into fine half-rings, are cooked slowly in 50g of lard or butter over moderate heat for 30-40 minutes, stirring occasionally, until uniformly golden and sweet but not dark brown. The slow cooking transforms the onions’ harsh sulphur compounds into complex sweet, savoury, and umami flavours through caramelisation and the Maillard reaction. A tablespoon of flour is stirred into the cooked onions (it will thicken the custard slightly), followed by the custard mixture: 200ml crème fraîche, 2 whole eggs, salt, pepper, and a generous grating of nutmeg. The base can be either a pâte brisée (for a crisp, shortcrust version) or a bread dough (pâte levée, for a softer, more substantial base that is the traditional winstub style). The base is rolled to fit a 28-30cm tart tin, the onion-custard mixture spread evenly, and optionally topped with a few thin strips of smoked bacon (lardons fumés). Baking at 200°C for 25-30 minutes until the custard is set and the surface has golden-brown patches. The tarte is served warm, never hot (the custard is best at warm room temperature when its texture is at its silkiest), cut into generous wedges with a green salad dressed in a simple vinaigrette. The contrast between the sweet, savoury onions and the tangy crème fraîche custard is the essence of the dish.
Onions sliced fine and cooked slowly 30-40 minutes until golden. Flour stirred into onions before custard. Crème fraîche and egg custard. Bread dough base traditional, shortcrust acceptable. Optional smoked bacon on top. Serve warm, not hot.
Add a teaspoon of caraway seeds to the onions during the last 5 minutes of cooking for authentic Alsatian flavour. If using bread dough, let it proof slightly in the tart tin for 15 minutes before filling for a puffier base. The tart reheats beautifully at 170°C for 10 minutes the next day.
Rushing the onion cooking, leaving them crunchy and sharp. Too much custard, making the tart wet and heavy. Oven temperature too low, producing a pale, flabby tart. Using sweet onions (they lack the complexity of yellow onions when caramelised). Serving straight from the oven when the custard is still liquid in the centre.
La Cuisine Alsacienne (Simone Morgenthaler)