Cook the potatoes whole without peeling. When they are done, let them cool and then peel and mash. Add the wheat groats, oil, flour and eggs and knead well – it should be hard, not sticky. Fry the bread crumbs in a pan (without oil!) until golden and set aside. Mix with the sugar.
Boil some water in a shallow and wide pot. Take a little dough at a time and place around a plum to make a little ball. Put them in the boiling water – you will realise they are done when they start floating. Remove them with a latticed spoon and roll them into the mixed bread crumbs and sugar.