Mix flour, fat, 1 egg, 1/2 tsp salt and 2-3 tbsp ice-cold water to a smooth dough. Cover and chill for about 30 minutes.
In the meantime clean, wash and cut the leek into pieces. Cover and steam briefly in little boiling salted water. Sprinkle prawns with lemon juice. Grate cheese roughly. Mix 4 eggs, sour cream and 2/3 of the cheese. Season with salt, pepper and nutmeg
Roll out the dough to the size of the tin and press down firmly at the edges. Fill the leek and prawns into the tin. Pour egg milk over them. Sprinkle with sesame seeds and rest of cheese. Bake in the preheated oven (electric cooker: 200 °C/circulating air: 175 °C/gas: level 3) for 45-50 minutes. Garnish with dill