For the dough mix flour and yeast. Peel, wash and finely grate 500 g potatoes. Put into a large bowl. Bring milk to the boil, pour over the potatoes. Add 1 tablespoon salt, flour mixture and egg. Knead everything with the dough hooks of the mixer to a smooth dough. Cover and leave to rise in a warm place for about 2 hours.
For the topping, stir cream cheese and sour cream until smooth. Season to taste with salt and pepper. Clean and wash the leek and cut into rings. Peel, wash and thinly slice the remaining potatoes.
Grease a fat pan (approx. 32 x 39 cm) and dust with flour. Press the dough into it evenly. Spread with horseradish cream. Spread leek and potato slices on top. Leave to rise for another 30 minutes. Season with salt and sprinkle with oil. Bake in the preheated oven (electric cooker: 225 °C/circulating air: 200 °C/gas: see manufacturer) for about 30 minutes. Cover with bacon.