In a large skillet, heat the skillet over medium heat and add the diced bacon. Fry until crispy and golden, about 5-7 minutes, stirring occasionally. Remove the bacon and set aside, keeping the rendered fat in the pan.

In the same skillet with the bacon fat, increase the heat to medium-high and add the diced chicken. Season with salt and pepper and cook for about 6-8 minutes until browned and no longer pink. Transfer to a bowl and set aside.

Lower the heat and add butter, leeks, and mushrooms to the skillet. Sauté for 5-6 minutes until leeks are translucent and mushrooms are tender. Add fresh herbs and cook for an additional minute.

Sprinkle flour over the vegetables and mix well, cooking for 1-2 minutes without browning. Gradually pour in chicken stock while stirring to prevent lumps. Add cream and simmer for about 5 minutes until the sauce thickens.

Add the cooked bacon back into the skillet and mix thoroughly. Allow to simmer for another 2-3 minutes, then remove from heat and let cool slightly.

Preheat oven to 140°C (284°F) and grease a pie dish. Roll out one sheet of puff pastry to line the dish and pour in the chicken filling. Roll out a second sheet and place it on top, sealing the edges and cutting vent holes.

Brush the top with a beaten egg or milk for a golden crust, ensuring steam can escape.

Bake for 40-45 minutes until the crust is lightly golden. Increase temperature to 180°C (356°F) and bake for an additional 15 minutes for a crisp finish.