Add the sugar and salt to the flour, mix well, add the milk and melted butter, mix with a spatula and knead by hand for about 5 minutes.
Place the dough in a bowl, cover with cling film and place in the fridge for about 30 minutes.
Chop the potatoes and onions and fry them in a pan. Season with salt and pepper and sauté until the potatoes are half cooked. Pour into a bowl and let cool.
Roll out the dough with a rolling pin and spread half of the mozzarella on top. Then add the fried potatoes and onions and the remaining mozzarella.
Roll out the dough, turn it upside down, roll out with a rolling pin, place on a hot tray, cover with a lid and put in the oven.
When the background color turns brown, turn over and fry. If the color is moderate, remove it.(cook about 8-10 minutes)