Thickly slice chicken thighs. Cut slices in half so you have large chunks of meat.
Heat a large frying pan or heavy-based saucepan with 2 TBSP of the oil. Brown the chicken in two batches until lightly golden, then removed and place in a bowl. Add chicken bacon to the pan and saute for 2-3 minutes until lightly golden. Add to the chicken and set aside to cool.
Add remaining oil to the pan. Add leek, cover and saute for a couple minutes until softened. Remove lid and cook the leek until the juices have evaporated. Add garlic and marjoram and saute for a minute. Add to the chick, cover with a paper towel and set aside.
Meanwhile prepare bechamel sauce: melt butter in a saucepan and sprinkle in the flour and stir. Cook for 2 minutes over gently heat. Add salt and pepper and stir well.
Continue stirring and add the milk slowly, 1/2 cup at a time. Stir until the sauce begins to thicken and is smooth. Simmer on low heat for 2 minutes then stir in cheese and mustard. Stir well, simmer for 2 minutes then removed from heat. Spread across a shallow dish to assist cooling. The sauce MUST be cold before continuing.
Brush a 2cm deep, 26-28cm square, or round pie dish with melted butter. Brush one sheet of filo with butter and line the dish (Butter side up). Repeat to make 8 layers. Keep pastry covered with a clean damp cloth while layers to keep it from drying out.
Take the bowl of cooled chicken, bacon and leek and tip into a sieve paced over a bowl. drain away the excess cooking juices and discard. combine with the cooled bechamel sauce in a large bowl and mis well with a fork. spread evenly into the filo base.
Butter and layer the remaining 8 pieces of filo pastry to make the lid. Brush pie edges with milk then press the pastry lid on and shap it to the ede of the pie tin, trimming off any excess pastry with kitchen scissors or a small knife. Brush with remiaing butter and sprinkle with seeds.
Bake 200 degrees C for 35-40 minutes.
A baking tin will transfer heat better than a ceramic dish and will result in a pie with a well-cooked base. Therefor if using a ceramic dish, increase cooking time.