In a greased 3-quart baking dish, arrange layers of the diced chicken seasoned with salt and pepper and the spinach seasoned with the rosemary. Spread 3/4 cup of cheese over the chicken-spinach mixture, poking some of the cheese into the chicken and spinach mixture.
Heat the butter and stir in the flour to make a smooth paste. Stir in the half and half and the milk a little at a time, and when it is smoothly blended, stir in the seasonings. Add the remaining 3/4 cup of cheese and simmer about 4 minutes. Spoon over the casserole.
Trim the bread slices to fit the top of the baking dish and toast them on one side. Fit the bread, untoasted side up, over the casserole and spread each slice with a generous amount of soft butter.
Bake in a preheated 350° F. oven for about 20 minutes, or until the bread top becomes crusty.
Recipe Adapted from (Chicken and Spinach Act II) "The LL Bean Book of "New" New England Cookery"