1. Grease the inside of a slow cooker with butter or non-stick spray.
2. Place the bread cubes in the slow cooker and scatter the sliced bananas and chopped walnuts evenly over the bread.
3. In a medium bowl, whisk together the eggs, milk, heavy cream, sugar, vanilla extract, cinnamon, and nutmeg until well combined.
4. Pour the egg mixture evenly over the bread, bananas, and walnuts in the slow cooker, ensuring that all the bread is coated.
5. Cover and cook on low for 4 to 5 hours, or until the bread is puffed up and the eggs are set.
6. Serve the casserole warm, garnished with additional banana slices and walnuts if desired.
