Tarte à l'oignon or Zewelwaï is a savoury tart with a baked filling of onions and cream. It is a speciality of the French region of Alsace, and may be served hot, warm or at room temperature. It is typically served as a starter.

Onion pastries are familiar in many French regions and elsewhere, including the Provençal pissaladière, Flemish flamiche and from Britain the Lancashire Butter pie.^[1] Elizabeth David singles out Zewelwaï as "the famous Alsatian speciality ... a truly lovely first course".^[2]

The onions are thinly sliced and, in most versions, slowly cooked in fat. Different cooks and writers specify various fats, including butter and oil (David),^[2] lard (Jane Grigson),^[1] beef dripping (Felicity Cloake), olive oil (Gilles Pudlowski),^[1] butter (André Soltner),^[3] and goose fat (Anne Willan).^[4] Larousse Gastronomique's Treasury of Country Cooking recommends simmering the onions in water;^[5] Gabriel Kreuther adds the onions raw to the filling.^[6]

The mixture to which the onions are added may contain cream, crème fraîche, whole eggs, egg yolks, and bacon.^[1] A variant made in the early months of the year, Zewelwaï printanière, substitutes spring onions for the usual brown ones.^[4] Most cooks blind bake the pastry case before adding the filling,^[1][4] although David does not.^[2] The pastry is generally shortcrust or a variant of it,^[1] but Larousse specifies puff pastry.^[5] The filled pastry case is then baked.^[1]

The cooked tart may be served hot,^[2][5] warm,^[1][3][6] or at room temperature.^[3][6]