In a small saucepan, cook spinach according to package directions; drain well.
In a medium sauce pan, make a white sauce; saute scallions in the margarine over low heat until tender. Stir in flour, adding milk all at once. Cook over low heat, stirring constantly until bubbly. Boil and stir 1 minute more until smooth and thickened.
Combine cracker crumbs and cheese. Dip chicken breast halves in crumb mixture to coat lightly. Arrange breast halves in a 13x9x2 inch baking dish. Seal leftover crumb mixture in a 1 qt bag (I save some to the side instead).
Stir spinach and ham into white sauce, spoon sauce over chicken breasts. Allow to cool and cover baking dish with foil, attaching bag of crumb mixture to side of dish. Freeze until needed.
To prepare for serving, thaw dish and bake covered in a preheated 350 oven for 60-75 minutes. Uncover and sprinkle top with crumb mixture. Bake for 10 minutes more.