1. Preheat oven to 350 degrees. In a mixing bowl beat eggs and egg whites with milk, mustard, and salt and pepper; set aside.
2. Place half the bread slices in a 13x9x2 inch baking dish coated with nonstick cooking spray.
3. In a skillet coated with nonstick cooking spray, saute the mushrooms, garlic, and onion until tender. Add the spinach and flour, stirring to mix well. Season with salt and pepper to taste.
4. Spread mixture over bread. Sprinkle with 1 cup cheese. Top with remaining bread. Sprinkle with remaining 1/2 cup cheese. Pour egg mixture over casserole and refrigerate two hours or overnight.
5. Bake for 40 to 50 minutes or until puffed and golden.