Preheat oven to 350F°. Slice banana quick bread into 12 slices. Melt butter in 13x9x2-inch baking pan. Sprinkle sugar over butter; place 6 bread slices in pan.
Whisk eggs, milk, vanilla, cinnamon and nutmeg together until well blended. Pour half of egg mixture over bread slices. Add second bread layer; top with remaining egg mixture.
Cover tightly, refrigerate 1 hour or overnight. Bake 35-40 minutes or until knife inserted into center comes out clean. Let sit 5 minutes; top with powdered sugar or maple syrup, if desired.