Trim spinach, remove the hard stems and rinse the spinach thoroughly. Drain spinach and coarsely chop. Peel onions and garlic and dice. Heat olive oil in a large saucepan. Sauté onions and garlic until soft. Add spinach and wilt over high heat, cook until the liquid has evaporated. Rinse dill, shake dry and chop. Crumble the feta cheese. Mix dill and feta with the spinach. Season with pepper and a little salt. Whisk eggs and stir into the spinach.
Warm the milk and butter in a pan over low heat. Separate filo sheets and sprinkle with water. Cover with a damp cloth. Line a greased dish with a sheet of dough so that the dough hangs over the edge. Brush with the milk-butter mixture. Cut dough sheets in the size of the pan and lay on the pan. Brush with the milk-butter mixture. Distribute spinach mixture on top. Fold overhanging dough inward. Cut another dough sheet the size of the pan and place it on the spinach. Push down on the edge. Brush the dough with the milk-butter mixture and sprinkle with sesame seeds. Bake in preheated oven at 200°C (gas mark 3-4, fan: 180°C) (approximately 400°F/convection 350°F) for 35-40 minutes.