Boil the potatoes in salted water and then mash them, add butter, salt, 2 tablespoons of milk and mix.
When the potatoes have cooled a little, add the egg, ham, cheese and onion. Stir and form balls from the mass, which you coat in egg, which you beat with a fork and add a little milk and then coat the balls in breadcrumbs. If you want a thicker coating, coat it several times in egg and breadcrumbs.
Fry in oil until golden. For a healthier, less greasy alternative, choose dry oven baking.