Prepare pastry for a single crust pie. Prick bottom and sides well with fork. Bake at 450 degrees for 10-12 minutes. Remove from oven and reduce temperature to 350 degrees.
Cook spinach according to package EXCEPT omit salt. Drain well, pressing out excess water. Combine eggs, mushroom soup, flour, horseradish and mustard; stir in drained spinach.
Spread hash in baked pastry shell; spoon spinach mixture over. Bake, uncovered for 45 minutes. Combine cheese and pimento; sprinkle over pie. Bake additional 2-3 minutes. Let stand before serving.