Put the mince in a bowl. Chop the onion very finely and add it to the mince. Add the egg, breadcrumbs and season as desired. Knead the mince and let it sit for 30 min.
In the meantime, peel the potatoes, wash and boil them. Drain out most of the water, leaving 2/5 cup (100 ml) of it with the potatoes. Heat the milk to boiling point. Mash the potatoes, while adding hot milk to them to obtain the desired consistency.
Add the butter, salt and stir. Pour the mashed potatoes into an oiled tray. Form small round meatballs from the mince and fry them. Arrange the meatballs over the mashed potatoes, pressing on them lightly to make them sink in halfway.
Cover with grated cheese on top. Bake in a preheated 392°F (200 °C) oven until the cheese crisps.