Knead the dough. Combine milk, egg, salt and sugar.
Melt the butter and pour into the milk-egg mixture.
Pour the sifted flour, mix and knead the dough.
Prepare the filling. Boil the potatoes. Remember with milk and butter. Sauté the onions and add in mashed potatoes. Close the lid so that it does not cool.
After 30 minutes, divide the dough into equal pieces (10-11 pieces). Put the filling on the half of the tortilla and cover with the second half.
Fry the cakes in a heated, dry pan with a thick bottom.