Defrost the spinach. Peel onions and garlic. Chop the onions finely, press the garlic through a garlic press. Heat the oil in a pot and fry the onions and garlic.
Add the spinach and steam covered for about 3 minutes. Remove the filo pastry from the refrigerator and let it rest at room temperature for about 10 minutes. Season spinach with salt, pepper and nutmeg, drain well on a sieve and let cool down.
Wash the herbs, dab dry, pluck leaves or flags from the stems and chop roughly. Put some parsley aside for sprinkling. Crumble the feta cheese. Squeeze the spinach thoroughly and chop coarsely.
Mix with breadcrumbs, feta cheese, parsley, dill, eggs and milk, season with salt, pepper and nutmeg. Melt the butter. Roll out the filo pastry. Line a greased square form (20 x 20 cm) dusted with flour with a sheet of dough until over the edge.
Spread the dough thinly with some butter. Place 4 more leaves on top and brush each with some butter. Fill with the spinach mixture and place the remaining sheets of pastry on top as well, brushing with the remaining butter.
Cut the upper dough-sheets to the size of the mould, fold the protruding lower dough-sheets towards the middle and press them on. Bake in the preheated oven (electric range: 175 °C/ convection oven: 150 °C/ gas: level 2) for 25-30 minutes.
Mix the egg yolk and 1 teaspoon of water. After about 20 minutes, brush the top of the dough with it and sprinkle with sesame seeds. Cut the pita warm into pieces and sprinkle with parsley.