Roll half of pastry to 1/8-inch thickness on a lightly floured surface; fit into a 9-inch pie plate. Set aside.
Combine apples and lemon juice in a large mixing bowl.
Combine sugar, flour, cinnamon, nutmeg, and salt, mixing well. Spoon over apple mixture, tossing gently. Spoon filling evenly into pastry shell, and dot with butter.
Roll remaining pastry to 1/8- inch thickness, and place over filling. Trim edges; seal and flute. Cut slits in top crust to allow steam to escape. Cover edges of pie with aluminum foil. Bake at 450° for 15 minutes. Reduce heat to 350°, and bake an additional 35 minutes.