In a large pan, brown chicken on high heat with olive oil, season with salt and pepper. Remove and set aside.
In the same pan, add leeks and garlic. Cook on until soft. About 5 minutes. Then,
add potatoes and flour. Mix well.
Slowly stir in chicken stock. Add dijon mustard, thyme and browned chicken, mix
and simmer for 10 minutes.
Pour heavy cream into the pan and mix well. Taste and adjust for salt and pepper.
Transfer filling into a pie dish. Lay puff pastry over the pie dish, trim edges, brush with egg wash and season with sesame seeds.
Bake on the middle shelf of the oven for 25 minutes on 210°C/410°F or until puff pastry is flaky and deep golden.