Spread bread lightly on both sides with butter. Place bread and cheese-spinach mix in layers in oblong Pyrex pan (13 x 9 x 1 3/4) as follows: Place 6 slices bread on bottom of pan, spread 1/2 cheese- spinach mix over bread. Make a third layer by using the 6 remaining slices. Cover with mix.

Cut remaining 6 bread slices diagonally into 1/4's. Top casserole with overlapping triangles and remaining shredded cheese. Beat together eggs, milk, mustard and seasoning. Pour carefully over all. Cover. Refrigerate at least 1 hour or overnight.

Bake 350 degrees for about 60 or 65 minutes or until knife is clean. Let stand 15 minutes. Cut in squares. Serves 8-10.