How to Make It
Arrange bread cubes in a 13- x 9-inch glass or ceramic baking dish coated with cooking spray. Using a fork, mash banana in a large bowl until smooth. Whisk in milk, eggs, egg whites, maple syrup, vanilla, cinnamon, and salt until combined. Pour mixture over bread cubes; toss to coat. Sprinkle berries over mixture. Cover with aluminum foil; chill 8 hours or overnight.
Preheat oven to 375°F.
Combine walnuts and brown sugar in a bowl. Add butter; using your fingers, mix until crumbly. Remove foil from chilled bread mixture; sprinkle bread mixture with walnut mixture. Replace foil cover.
Bake at 375°F until casserole is set, about 30 minutes. Remove foil; bake until golden brown, about 10 minutes. Let stand at room temperature 5 minutes.