Peel and roughly dice the potatoes and place in a saucepan with the chopped onion. Cover with water and cook for 20 minutes. Then pour off the cooking water - collect a little bit of it.
Mash the potato and onion cubes and whisk to a compact mass (if the mass is too firm, add some of the cooking water).
Pour the cream into a baking dish and keep it warm in the preheated oven at 100 ° C. Carefully shape balls out of the potato mixture with an ice cream scoop and place in the baking dish.
Lightly brown the butter in a pan, pour over the potato balls and sprinkle with breadcrumbs and serve immediately.