Heat oven to 425 degrees F. Place apples in large bowl; mix sugars, cornstarch or flour, cinnamon, nutmeg and salt in a small bowl; sprinkle over apples; toss gently to mix. Let stand until a little juice forms, about 10 minutes.
Meanwhile, prepare piecrust mix, following package directions. Roll out half of dough to a 12-inch round on a lightly floured surface; fit into a 9-inch pie plate. Trim overhang to 1/2 inch.
Pile apple mixture into pastry; dot with butter or margarine. Moisten edge of bottom pastry with water. Roll out remaining pastry for top to a 12-inch round; carefully drape over filling. Trim overhang to 1 inch; turn edges under and press together to seal. Flute decoratively. Cut vents in pastry with paring knife.
For a crispy-sugary top, brush top of pastry with a little water or milk and sprinkle lightly with sugar.
Bake at 425 degrees F for 40 minutes or until juices bubble through slits and apples are tender. If edge is browning too fast, cover with a narrow strip of foil. Cool completely or serve slightly warm.