Thaw spinach and drain well. Roll out half the pastry and line 9-inch pie pan. Separate 1 egg, then brush egg white on pie shell. Pour excess white into large bowl; add yolk and remaining 2 eggs; set aside.
In skillet saute onion in butter until tender. Meanwhile beat eggs; stir in cheese, spinach, salt, pepper, and onion. Turn into pie shell. Roll out remaining pastry, fit over pie and crimp edge.
Bake in preheated 425 degree oven 30 minutes or until crust is golden. Cool on rack 10 minutes. Cut in wedges to serve. Serves 6-8.
