|Butter a casserole very well as this will stick if you don't. Lay down slices of bread to cover bottom of dish. Put a slice of cheese, then the spinach, then a slice of ham, then another slice of cheese, and finally another slice of bread.|
Beat about 1 cup of milk, 1 large egg, 1 Tblsp. mustard, 1/4 cup onion, and 1 tsp. season salt per sandwich in a blender until the onion has all become one with the liquid. Add horseradish if desired and blend well.
Pour the liquid over the sandwich and let it set for an hour or so if you have the time. This is a good make ahead recipe. Bake covered but so that the cover doesn't touch the casserole at 350* F. until a tooth pick comes out clean. Brush the top with butter, remove the cover and bake until browned if desired.