Alsatian Onion Tart

Alsace is the homeland of the onion tart—a rich combination of cooked onions, bacon, and a savory custard in a crumbly crust. Tarts like this are made all over the region and are also a traditional Carnival dish in nearby Basel, in Switzerland. I like to use lard in the crust for a rich and crumbly texture, though you may substitute butter if you like. I prefer white onions for the filling because they are a little less sugary-sweet after they are cooked.