Leave the bacon in a pan, add the onions, garlic and mushrooms, let them collapse and reduce until there is hardly any liquid left.
Let cool and mix in a bowl with cream cheese, parmesan and herbs. Season with salt and pepper if necessary.
Boil the potatoes until soft, press through, mix with egg yolk, flour and nutmeg and knead well until a smooth dough is formed. Form small balls on a floured work surface with floured fingers and make circles as thin as possible. Put a good portion of the filling in the middle and fold it up in a crescent shape. Press the edges well.
Fry until golden brown on both sides in plenty of fat.
A fresh salad and a fruity tomato dip go well with it. (That`s how we had it, but I could also imagine herbal quark or something similar.)
Leftovers of the filling taste great as a spread! You can also eat the cookies wonderfully cold, e.g. as finger food for a party.