Beat egg whites into snow. Mix the egg yolks with soft butter until foamy and then add the flour, potato flour and the spices and mix well.
Knead the cooked, grated potatoes with the butter-egg mixture. Use the mixer to stir about half of the egg whites into the potato mixture and fold in the rest. The result is a heavy dough.
Peel the onion and garlic and cut into small cubes. Also dice the mushrooms. Heat butter and oil in a pan and fry everything in it until the liquid has evaporated and the mushrooms begin to turn color.
Grease a springform pan (20 cm) and distribute a third of the potato mixture evenly in it. Goes great with moistened hands.
Let it brown under the grill for approx. 10 - 12 minutes. Then spread half of the mushrooms on top and cover them with another third of the potato mixture. Put it back under the grill. Do the same with the rest. Only now the surface should not be quite so brown.
Remove the potato cake from the mold and cut into the desired portions. Serve immediately.
Note 7: You can also prepare this potato cake very well. The individual portions are then fried in a butter / oil mixture before serving.
* Link to spice mixes: mushroom salt, the second version