Combine flour and salt; cut in shortening. Combine 1 egg, cold water, and vinegar; sprinkle over surface, stirring until dry ingredients are moistened. Shape into a ball; chill. Saute onion in margarine in a skillet 5 minutes. Stir in chicken and next 6 ingredients. Bring to a boil; cover, reduce heat, and simmer 20 minutes. Combine cornstarch and 2 tablespoons water; add to chicken mixture. Cook, stirring constantly, until mixture boils. Remove from heat; stir in soup, sour cream, and cheese.
Roll half of pastry to 1/8-inch thickness. Fit into a deep 2-quart casserole. Spoon chicken mixture into casserole. Roll remaining pastry; place over chicken mixture. Trim, seal, and flute edges. Cut slits in pastry. Combine 1 egg and milk; brush over pastry. Bake at 400° for 30 minutes.