Cook the vegetables until they are very soft. This usually takes about eight to ten minutes. Stir them occasionally so they do not burn.

Whisk in the flour to coat the vegetables. Cook this for two minutes to remove flour taste. Slowly pour in the warm chicken stock now.

Stir the mixture until the sauce gets thick. Add the heavy cream and the fresh thyme. Season it well with salt and black pepper.

Fold in the cooked chicken and frozen peas. Stir gently to combine all the filling ingredients. Pour the hot mixture into your baking dish.

Unroll the thawed puff pastry on a board. Place it carefully over the top of the dish. Trim the edges to fit the dish perfectly.

Cut a few small vents in the pastry top. This allows the steam to escape while baking. Brush the dough with a beaten egg wash.

Place the dish in the hot center oven. Bake for twenty-five to thirty minutes total. The pastry should be very high and golden.