First knead the flour with 1/2 teaspoon salt, the butter and 5 tablespoons ice-cold water to a smooth dough. Shape it into a ball, wrap it in cling film and refrigerate for about 30 minutes.
Cut the leek into fine rings and finely dice the ham. Mix the sour cream with the eggs and flour and season with salt, pepper, paprika and nutmeg. Now add the leek, ham and grated cheese and mix everything well. Carefully line the bottom and the edge of a greased quiche tin (or 28 springform pan) with the dough and prick several times with a fork. Spread the icing on top and bake in a preheated oven at 180 degrees for approx. 45 - 50 minutes until golden brown.
Check the degree of browning and, if necessary, turn the heat down slightly towards the end of the baking time. Let cool a little and serve lukewarm.