Melt half of the butter in a large skillet over medium-high heat. Season the chicken thighs with salt and pepper, then sear them in the hot pan until golden brown on both sides, about 5 minutes per side. Remove the chicken from the pan and set aside to cool.

In the same skillet, add the chopped bacon and cook until crispy, about 4 minutes. Stir in the sliced leeks, celery, and minced garlic, cooking gently until fragrant, about 5 more minutes.

Pour in the dry white wine and let it simmer until reduced by half, about 3 minutes. Add the remaining butter and sprinkle the flour over the mixture, stirring constantly to create a roux.

Gradually whisk in the chicken stock while stirring to prevent lumps. Continue to mix until the sauce thickens, about 3-5 minutes. Stir in the Dijon mustard, cream, thyme, bay leaves, and sliced chicken.

Preheat your oven to 400°F (200°C). On a lightly floured surface, roll out your puff pastry to fit your pie dish. Pour the cooled filling into the pastry-lined dish, then cover with another layer of puff pastry.

Seal the edges and use a knife to cut slits for steam to escape. Brush the top with an egg wash and bake for about 45 minutes until golden brown.