Cut the cold potatoes into slices.
Chop the onion and fry in a little fat until golden brown.
Cut the sausage into small pieces or strips and the hard-boiled eggs into slices.
Butter a baking dish and place the potatoes, sausage and onion alternately in the dish, add a thin layer of sour cream and repeat until all the ingredients are used up.
Finally, pour the remaining sour cream on top and bake in the oven at 170 – 180 °C (convection oven) for about 20-25 minutes.
Arrange the Hungarian potato casserole and serve.