Spray 13x9-inch (3-quart) glass baking dish with cooking spray.
In large bowl, stir brown sugar, butter, and walnuts until smooth. gently stir in bananas. spoon mixture into baking dish.
Arrange bread on banana mixture.
In medium bowl, beat eggs, milk, cinnamon, nutmeg and vanilla with wire whisk until well blended. pour over bread. cover tightly; refrigerate at least 1 hour or overnight.
Heat oven to 350°f. uncover; bake 50 to 55 minutes or until knife inserted in center comes out clean. cool 10 minutes., serve portions upside down, spooning sauce from bottom of dish over each serving. sprinkle with whip cream