Preheat oven to 350 degrees. Melt butter in saucepan and cook chopped onion until tender. Stir in celery and carrots and cook for 2 minutes. Stir in flour and cook for 2 minutes. Add chicken stock and bring to a simmer. Add potatoes and simmer until tender. Stir in chicken, parsley and peas. Pour mixture into casserole. Top with pie crust and brush with egg. Bake for 30 minutes until crust is golden.