Prepare the dough by combining flour, sugar, and salt in a food processor.
Add cubed butter and pulse until it resembles small peas.
Gradually add ice-cold water, mixing until the dough comes together.
Wrap the dough in plastic wrap and chill for at least an hour.
Preheat your oven to 375°F (190°C).
Roll out the chilled dough on a floured surface to fit your pie dish.
Line the pie dish with the dough if you prefer a bottom crust.
In a large skillet, melt some butter and sauté onions, carrots, and celery until tender.
Add peas, thyme, and chicken. Mix well.
Sprinkle in flour, stirring constantly for one minute.
Pour in the chicken stock and cream, stirring until the mixture thickens.
Add salt and pepper to taste.
Transfer the filling into the pie dish.
Roll out remaining dough and place over filling.
Cut slits to allow steam to escape.
Bake in the preheated oven for 35-40 minutes or until the crust is golden brown.
Let it cool for 10 minutes before serving.