Place 6 bread slices on the bottom of the casserole dish. Sprinkle with bacon, green onions, cheese and mushrooms. Cover with remaining 6 slices of bread.
In a bowl, stir together milk and eggs. Pour over bread.
Cover and refrigerate for 1 hour or overnight.
Preheat oven to 350°F. Bake for 1 hour, uncovered. Garnish with additional green onions, if desired.