Preheat oven to 325°.
Steam spinach until tender (6 to 8 minutes).
Scald milk (150°) in a heavy saucepan.
Combine cheese, flour, milk, salt, and pepper until the cheese is coated with flour.
Stir cheese mixture into the scalded milk; add steamed spinach, and finally stir in the beaten eggs and onion.
When thoroughly combined, pour mixture into pie crust and bake for 30 to 40 minutes or until a table knife inserted in the center comes out clean.
This quiche can be reheated after it has been baked.
Garnish with thin slices of tomato.