Preheat the oven to 375ºF. Mash the bananas well with a fork.
Add the mashed bananas to a large bowl along with the brown sugar, egg, salt, vanilla, baking powder, cinnamon, nutmeg, and salt. Whisk until the ingredients are combined.
Add the milk and whisk until combined again. Finally, stir in the rolled oats and chopped walnuts.
Pour the oat mixture into a 9x9-inch casserole dish and transfer to the preheated oven. Bake for 40-45 minutes, or until it is golden brown on top and around the edges. Serve warm.