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.
Pictures shown below is a portion of Butterkuchen which is an example of Blechkuchen.
List of books: Blechkuchen