Grease an 11×7 non-stick baking dish with cooking spray. Add the mashed bananas to the dish and mash them further with a fork.
Combine all the ingredients in the prepared baking dish – oats, diced apples, maple syrup, baking powder, cinnamon, nutmeg, salt, milk, egg, melted butter, and vanilla extract. Stir everything together until well combined.
Bake for 25-30 minutes or until the oatmeal is set and golden brown on top.
Slice and serve with your choice of nuts and a drizzle of maple syrup if you like. Enjoy your delicious and nutritious breakfast!