Boil the potatoes in their skins until barely cooked, drain and cool under cold running water. When cool, peel and grate into a bowl.
Slice the onions in rings, sprinkle with salt, leave for 3 minutes, rub, rinse, squeeze out the excess water and add the onions to the potatoes. Add a pinch of salt, the cumin, red and black pepper and mix. Arrange in the middle of a serving dish. Surroun