Cut the onion into thin crescents and fry it in butter. Add the pre-boiled and mashed potatoes, while still warm. Stir and add all the other products and spices for the filling. Put 2 phyllo pastry sheets on the kitchen counter and cut them into 4 strips.
At the end of each, distribute some of the filling and roll them up lengthwise, finally sticking the pastry sheets together with a little water. Repeat this for all the remaining phyllo pastry and filling. Arrange the potato pastries in a tray with greased baking paper with the folded sides down.
Coat the pastries with beaten yolk and sprinkle them with black cumin, or use poppy or sesame seeds if you don't have that. Put them to bake in a preheated 392°F (200 °C) oven for about 15 minutes or until they acquire an appetizing golden color.