In a large bowl, combine the potatoes, contents of sauce mix and water. Stir in the peas and carrots, ham, soup, mushrooms, milk, sour cream and pimientos. Transfer to an ungreased 2-qt. round baking dish.
Roll out pastry to fit top of dish; place over potato mixture. Flute edges; cut slits in pastry. Bake at 400° for 40-45 minutes or until golden brown. Let stand for 5 minutes before serving. Yield: 4-6 servings.