Place 4 of the bread slices on bottom of greased 8-inch square baking dish; top with bacon, mushrooms, onions and cheese. Cover with remaining 4 bread slices.
Beat eggs and milk with wire whisk until well blended; pour over bread; cover. Refrigerate at least 1 hour or overnight.
Bake, uncovered, in preheated 350F-oven for 1 hour or until center is set.