Prepare the Filling
1-In a large skillet or frying pan,: In a large skillet or frying pan, heat the olive oil over medium heat. Add the chopped onion, diced carrots, and diced celery. Cook, stirring occasionally, for about 5 minutes until the vegetables begin to soften.
2-Add the minced garlic, fresh thyme leaves,: Add the minced garlic, fresh thyme leaves, and bay leaf to the pan. Cook for another 2 minutes until fragrant.
3-Season the chicken breast pieces with salt: Season the chicken breast pieces with salt and black pepper, then add them to the pan. Cook, stirring occasionally, until the chicken is browned on all sides.
4-Sprinkle the flour over the chicken and: Sprinkle the flour over the chicken and vegetables, stirring to coat evenly. Cook for 1-2 minutes to cook out the raw flour taste.
5-Gradually pour in the chicken stock, stirring: Gradually pour in the chicken stock, stirring constantly to prevent lumps from forming. Bring the mixture to a simmer, then reduce the heat and let it cook for 5-7 minutes until thickened.
6-Stir in the frozen peas and heavy: Stir in the frozen peas and heavy cream. Cook for an additional 2-3 minutes until the peas are heated through and the sauce is creamy. Remove from heat and set aside.

Prepare the Pastry
7-In a large mixing bowl, combine the: In a large mixing bowl, combine the all-purpose flour and salt. Add the cold cubed butter to the bowl.
8-Use your fingertips to rub the butter: Use your fingertips to rub the butter into the flour until the mixture resembles coarse breadcrumbs.
9-Gradually add the cold water, one tablespoon: Gradually add the cold water, one tablespoon at a time, mixing with a fork until the dough comes together into a ball.
10-Turn the dough out onto a lightly: Turn the dough out onto a lightly floured surface and knead it gently until smooth. Wrap the dough in plastic wrap and refrigerate for at least 30 minutes before using.

Assemble the Pie
11-Preheat your oven to 400°F (200°C): Preheat your oven to 400°F (200°C).
12-Roll out the chilled pastry dough on: Roll out the chilled pastry dough on a lightly floured surface to about ¼ inch thickness.
13-Place the rolled-out pastry over the chicken: Place the rolled-out pastry over the chicken filling in the skillet or transfer the filling to a baking dish. Trim any excess pastry and crimp the edges to seal.
14-Use a sharp knife to make a: Use a sharp knife to make a few small slits in the top of the pastry to allow steam to escape during baking.
15-Optional: Optional: Brush the top of the pastry with beaten egg for a golden finish.
16-Place the skillet or baking dish in: Place the skillet or baking dish in the preheated oven and bake for 25-30 minutes until the pastry is golden brown and the filling is bubbling.
17-Remove the chicken pie from the oven: Remove the chicken pie from the oven and let it cool for a few minutes before serving.