Grind poppy seeds using a blade grinder or a poppy seed grinder.
Preheat the oven to 180 °C (355 °F), rack in the middle.
Using a pastry brush coat the bottom of 400 mm x 230 mm x 50 mm (15 3/4″ x 9″ x 2″) baking pan.
Line the bottom of the baking pan with the first filo pastry sheet. The filo sheet length will match the baking pan length. Since filo sheet is wider than the baking pan fold it back at the pan’s corner.
Brush or sprinkle the top of the filo sheet with vegetable oil, sprinkle evenly across the whole surface couple table spoons of ground poppy seeds. Sprinkle little granular sugar on top.
Line the baking pan with the second filo pastry sheet, starting from the end of the first sheet, and again fold the sheet at the pan edges in order to fit to the baking pan. Repeat with brushing with oil, sprinkling ground poppy seeds and sugar.
Repeat the whole process until all the filo sheets and ground poppy seeds are used up. The last stacked filo sheet will be without any filling, it will be empty.
With a sharp knife carefully cut the stacked pastry into 50 mm (2″) squares.
Bake for about 45 minutes or until golden brown. Prepare the syrup during baking time.
Laddle the syrup by pouring along the pastry cut edges. Spread lemon slices on top of it. Let it fully soak in, cool completely, the best if left overnight.
Syrup
Add sugar and water to a medium size sauce pan. Stir until sugar is completely dissolved. Over medium high heat bring it to a full boil and then add sliced lemon to it. Let it come to a boil again, keep it for couple of more minutes and then take off the heat.