• Heat the butter and fry the leeks over medium heat for 5 minutes. Add the mushrooms and fry for a few minutes. Remove and set aside.
• Melt the remaining butter and add the flour. Cook for 2 minutes, then remove from the heat. Add the milk a little at a time and stir well.
• Add the vegetables and chicken and stir well. Season with salt and black pepper. Spoon the mixture into a large pie dish.

To make the lattice top:

• Keep the pastry cold.
• Cut the pastry into strips, about 1,5cm wide. On a large piece of baking paper, lay strips down that will be as long and wide as the pie dish. Fold every second strip over backwards onto itself.
• Place a strip of pastry over the strips that are still flat, then fold the strips back over this strip. Lift the strips that are under the strip up and backwards and place a strip down over again.
• Continue lifting every second strip and placing strips across until the bottom is covered. Then do the same with the other side until you have a large solid sheet of woven strips. Refrigerate to keep the pastry cold.
• Slide it off the paper onto the top of the pie filling. Wet the edge of the pie dish and press down lightly to seal the edges. Cut off the extra pastry around the sides. Brush with egg wash, top with black sesame seeds and bake for 20-25 minutes at 180℃ until golden.