Roll half of pastry to 1/8-inch thickness on a lightly floured surface; fit pastry into a 1-quart casserole. Set remaining pastry aside.
Combine grapes and water to cover in a medium saucepan; cover and cook over medium heat 10 minutes or until grapes are soft. Drain off liquid, reserving grapes in saucepan. Add sugar, cornstarch, and lemon juice to saucepan, stirring well. Cook over medium heat, stirring occasionally, until thickened. Stir in butter; pour mixture into pastry shell.
Roll remaining pastry to 1/8- inch thickness; place over filling. Trim edges; seal and flute. Cut slits in top crust to allow steam to escape. Bake at 350° for 30 minutes or until lightly browned.