Swiss Onion Tart

This onion tart, a cheese-custard filled tart, and a thin onion soup are the main elements of the breakfast eaten at dawn in Basel for the first day of Fastnacht, their annual Carnival celebration. We would certainly think of it more as an appetizer or a brunch dish. The secret of a good onion tart is cooking the onions for a long time to fully develop their sweetness. The method here is geared toward first releasing, then evaporating, the high water content of the onions, so that you are l