Make sure you are starting with stale bread. If you haven't left your bread out for a few days, you can quickly dry out in the oven. See post for instructions on doing this.
In a large bowl, mash the bananas. Then whisk in the remaining ingredients for the french toast mixture. *Tip: Can add bananas and remaining ingredients to a blender.
Add stale bread to a greased 9x13 casserole dish and then pour the mixture evenly across the top. Use a spatula to mix together, making sure that all the pieces of bread are coated well.
Cover and refrigerate for at least 4 hours or overnight.
Once ready, preheat oven to 350 degrees F.
In a small saucepan make the topping. To start, add the butter and let melt.
Add brown sugar and whisk until it's a caramel like texture.
Stir in chopped walnuts, nutmeg and cinnamon.
Spread the mixture over bread mixture.
Bake for 35-40 minutes. Check at the 30 minute mark to see if it is browning too much. If it is, then cover with foil and let finish cooking.
Remove from oven and let rest for 5 minutes.
Top with powdered sugar if desired and serve with maple syrup and fresh fruit.
Store any leftovers in the fridge, covered, for up to 5 days.