In a large saucepan, combine the potatoes, broth, onion, butter, salt and pepper. Cook, uncovered, over low heat until potatoes are crisp-tender and broth is absorbed, about 20 minutes; drain if necessary.
Line a 9-in. pie plate with the bottom pastry. Spoon potato mixture into the crust. Roll out remaining pastry to fit top of pie. Trim, seal and flute edges; cut vents in top. Brush with milk.
Bake at 425° for 30-35 minutes or until golden brown. Serve warm.