In a large pot, soften the onion, carrot and celery in the remaining butter. Season with salt and pepper. Add the broth and potatoes and bring to a boil. Cover and simmer for about 10 minutes or until the vegetables are just tender. Add the butter mixture and boil gently, stirring with a wooden spoon. Add the milk and cornstarch mixture and bring to a boil. Adjust the seasoning. Stir in the chicken and peas. Cover with plastic wrap directly on the surface of the filling. Let cool while preparing the pastry.