Put both types of flour into a large bowl or kneading trough, and make a well in the middle.
Dissolve the yeast in the lukewarm water, and mix in enough flour to get a pancake-batter thickness.
Pour it into the well, and let it rise in a warm place for about 20-30 minutes.
Once risen, sprinkle the salt and the sieved potato over the flour, then add enough lukewarm water (carefully, so it's not too hot or it kills the yeast) to make the dough rather firm than soft.
Knead thoroughly - careful kneading matters a great deal here.
Cover and let rise for about 45-60 minutes, then work it through again, knead it, and put it in a basket or tin.
After another rise (about 30-40 minutes), bake in a preheated oven (starting at 220°C, then lowering to 190-200°C) for 1 to 1.5 hours depending on its size.
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.