Blechkuchen is the German word for sheet cake or a cake baked in cake tray/baking sheet. It is when the Batter or the dough is spread on the baking/cake tray/sheet then baked and then spreaded or filled with other ingredients. Moreover, Blechkuchen is so called as such because the cake is baked in a baking sheet/cake tray
Examples of famous sweet Blechkuchen from Germany are Donauwelle, Bienenstich, Butterkuchen, Zwetschgenkuchen and lot more considering that different kinds of fillings or toppings can be used. Zwiebelkuchen is a kind of savory "Blechkuchen".
Blechkuchen are generally rectangular in form and they are cut into squares when served.
Blech means "sheet" of metal and in German it refers to "metal baking sheet/cake tray" used to make their varieties of Blechkuchen.
Pictures shown below is a portion of Butterkuchen which is an example of Blechkuchen.
List of books: Blechkuchen