Preheat oven to 450 degrees F. Prepare Pastry for Single-Crust Pie. On a lightly floured surface, roll pastry into a circle about 12 inches in diameter. Line a 9-inch foil pie pan with pastry. Trim pastry to 1/2 inch beyond edge of pie plate. Fold under extra pastry. Crimp edge as desired. Do not prick pastry. Line the pastry shell with a double thickness of heavy foil. Bake for 8 minutes; remove foil. Bake for 6 to 8 minutes more or until pastry is golden. Remove from oven. Reduce oven temperature to 325 degrees F. (Pie shell should still be hot when filling is added; do not partially bake pastry shell ahead of time.)
Meanwhile, in a very large skillet, cook mushrooms and onion in hot oil until tender, stirring occasionally. Stir in spinach; set aside.
In a bowl, stir together eggs, half-and-half, salt, and nutmeg. Stir in the spinach mixture. In another bowl, combine shredded cheese and flour; add to egg mixture. Mix well.
Pour egg mixture into the hot, baked pastry shell. Bake in the 325 degree F oven for 50 to 55 minutes or until a knife inserted near the center comes out clean. If necessary, cover edge of crust with foil to prevent overbrowning. Cool completely on a wire rack. Cover and chill within 2 hours.