For the filling, chop the sauerkraut finely, the mushrooms into pieces, the cooked meat into pieces. Melt 1/2 the butter, add the mushrooms, sauerkraut, meat and close the lid of the pot.
Leave to saute until its sauce evaporates. Before taking the pot off the stove, add a pinch of rosemary and stir. Melt the rest of the butter and grease a tray. Place a phyllo pastry sheet in it and cut it to the size of the tray bottom.
Lay on the rest of the sheets on top, crisscrossing them with their edges spilling out of the tray. Mix the beaten egg with the yoghurt, feta cheese and baking powder.
Distribute 3-4 tbsp of the mixture on top of the sheets, add the filling, then another 3-4 tbsp yoghurt mixture. Cover the filling with the sheets' edges that are spilling out, smear melted butter on top.
Distribute the remaining yoghurt mixture on top and sprinkle grated cheese. Bake in a preheated 356°F (180 °C) oven about 30-40 min. to a nice crisp.