Preheat oven to 210°C/410°F.
In a large pan, brown chicken on high heat with 1 tablespoon of olive oil, season with salt and pepper. Remove and set aside. (note 2)
In the same pan, add leeks and garlic. Cook on medium-low heat until soft. About 5 minutes.
Keeping the heat at medium-low, add potatoes and flour. Mix well.
Pour chicken stock into the pan, roughly ⅓ at a time, mixing well with a spatula after each pour. This will help the sauce thicken smoothly without lumps.
Add dijon mustard, thyme and browned chicken, mix and reduce the heat down to low. Simmer for 5 minutes with the lid on then further 5 minutes with the lid off to evaporate excess liquids. Stir occasionally.
Meanwhile, take the puff pastry out of the freezer to thaw just in time for pie assembly.
After 10 minutes of simmering, pour heavy cream into the pan and mix well. Taste and adjust for salt and pepper.
Transfer chicken pie filling into a pie dish (or a small oven-proof pan like the pictures in this post). Brush the rims of the pie dish with a little oil.
Lay puff pastry over the pie dish, trim edges, brush with egg wash and generously season with sesame seeds. Make a little 'X' incision with a sharp knife in the middle of the pie to allow steam to escape whilst baking.
Bake on the middle shelf of the oven for 25 minutes or until puff pastry is flaky and deep golden.