- Preheat oven 350°F. Coat 9x9x2-inch baking dish with nonstick cooking spray.
- In large nonstick skillet coated with nonstick cooking spray, sauté mushrooms, onion, and garlic until tender. Add spinach, stirring until wilted, and add flour, stirring to mix well. Season to taste; set aside. Place half bread slices in prepared dish.
- Spread spinach mixture over bread and sprinkle with 3/4 cup cheese. Top with remaining bread and remaining 1/4 cup cheese.
- In a bowl, whisk together eggs, egg whites, milk, and mustard. Pour egg mixture evenly over the casserole, refrigerate 2 hours or overnight.
- Bake at 350°F 40-50 minutes or until puffed and golden.
Calories 210, Carbohydrates 26g, Cholesterol 62mg, Fat 5g, Fiber 2g, Protein 16g, Saturated Fat 2g, Sodium 352mg. Daily Values: . Percent Daily Values are based on a 2,000 calorie diet.