Remove papery outer skin from garlic (do not peel or separate cloves). Cut tops off of garlic bulbs; brush bulbs with 1 tablespoon oil. Wrap each bulb in heavy-duty foil. Bake at 425° for 30-35 minutes or until softened. Cool for 10-15 minutes., Meanwhile, place spinach in a greased 13x9-in. baking dish; sprinkle with 1/4 teaspoon each of salt and pepper. In a large skillet, brown chicken in remaining oil on both sides; place over spinach., In a large saucepan, melt butter. Stir in flour until smooth; gradually add milk. Bring to a boil; cook and stir for 1-2 minutes or until thickened. Stir in 2 cups cheese, nutmeg and remaining salt and pepper. , Transfer to a blender; squeeze softened garlic into blender. Cover and process until smooth. Pour mixture over chicken., Cover and bake at 425° for 30-35 minutes or until a thermometer reads 170° and sauce is bubbly. Uncover; sprinkle with remaining cheese. Bake 5 minutes longer. Serve with pasta. Sprinkle with tomato and parsley if desired. Freeze option: Substitute 5 ounces frozen chopped spinach that has been thawed and squeezed dry for fresh spinach. Cool unbaked casserole and sprinkle with remaining cheese; cover and freeze. To use, partially thaw in refrigerator overnight. Remove from refrigerator 30 minutes before baking. Preheat oven to 425°. Bake casserole as directed, increasing time as necessary to heat through and for a thermometer inserted in chicken to read 170°.