Prepare the Filling : Heat olive oil and butter in a large skillet over medium heat. Add the sliced leeks and cook for 5-7 minutes until softened. Add the minced garlic and cook for another minute until fragrant. Add the chicken pieces to the skillet and cook for 5-6 minutes until they are lightly browned and just cooked through.

Make the Sauce : Sprinkle the flour over the chicken and leek mixture and stir well. Gradually pour in the chicken broth, stirring continuously to prevent lumps. Add the heavy cream and milk, and stir until the sauce begins to thicken. Season with salt, pepper, thyme, and rosemary. Add the frozen peas (if using) and cook for an additional 2-3 minutes. Remove from heat and allow the filling to cool slightly.

Preheat the Oven

Preheat your oven to 375°F (190°C).

Assemble the Pie : Roll out one pie crust and place it in a 9-inch pie dish, trimming any excess. Pour the cooled chicken and leek filling into the crust. Place the second pie crust on top, sealing the edges by crimping with a fork or your fingers. Make a few small slits in the center of the crust to allow steam to escape.

Bake the Pie : Place the pie dish on the middle rack of the preheated oven. Bake for 25-30 minutes, or until the crust is golden brown and the filling is bubbling. If the edges of the crust brown too quickly, cover them with aluminum foil.

Serve : Let the pie cool for 5-10 minutes before slicing. Serve warm with a side salad or steamed vegetables for a complete meal.