Step 1

Cook vegetables and chicken: Bring a large pot of water to a boil. Season with salt and boil the diced potato, carrot, celery and chicken until tender, about 8-10 minutes. Drain and set aside.

Step 2

Make the roux: In the meantime, melt the butter in a deep, wide skillet over medium heat. Add the onion and cook until softened WITHOUT browning. Whisk in the flour and cook for one minute, whisking constantly. Pour in the chicken stock and milk, whisking carefully so no lumps can form.

Step 3

Cook the filling: Season the filling with salt, thyme and black pepper. Simmer until thickened, about 5 minutes.

Step 4

Finish the filling: Stir the potato mixture, chicken breast, peas and corn into the filling. Set aside to cool completely before adding to the pie crust.

Step 5

In the meantime, prepare the pastry: Use half of the pastry to fill a 9-inch pie plate. trim the edges and set the pie plate into the refrigerator to chill. Roll the remaining pastry into an about 10-inch circle. Place on parchment paper and cover with cling film or a clean kitchen towel. Chill in the fridge as well until the filling is ready.

Step 6

Finish the pie: Preheat the oven to 420°F. Place the cold filling into the pastry-lined pie dish. Place the pastry lid on top. Trim, seal and flute the edge. Cut slits into the top as air vents. Brush with beaten egg. Bake in the preheated oven for 35-40 minutes or until lightly browned. Allowed to rest for 10 minutes before slicing.