Cut the peeled potatoes into equal pieces. Boil the potatoes until tender in salted water. Drain.
Mash the potatoes and add the butter and milk. Season with nutmeg, pepper and salt.
Prepare the fill-in:
Chop the onion and the mushrooms finelly.
Melt the butter in a pan. Add the mushrooms and onion. Season with salt and simmer over low heat for 10 minutes, until the mixture is dark brown and dry (stir occasionally).
Put the mashed potatoes, egg yolk, flour and pepper in a bowl. Form small balls of equal size. Crush them into palm-sized discs. Arrange some of the filling on top and fold the edges over so that the filling is well coated.
Heat the olive oil in a pan. Fry 3-4 zrazy at a time over medium heat, four minutes on each side, until golden brown and crispy.
Serve with a dip or sauce of your choice, such as sour cream or tomato salsa.