01 - Preheat the oven to 350°F. Grease an 8x8-inch baking dish with cooking spray or butter.
02 - In a large bowl, whisk together the mashed bananas, eggs, milk, maple syrup, vanilla extract, and melted butter until well combined and smooth.
03 - In a separate medium bowl, combine the rolled oats, baking powder, cinnamon, nutmeg, and salt. Stir thoroughly to evenly distribute the spices and leavening agent.
04 - Add the dry ingredients to the wet mixture. Stir gently with a spatula or wooden spoon until just combined, being careful not to overmix.
05 - Pour the batter into the prepared baking dish. Use a spatula to spread the mixture evenly into the corners.
06 - Bake for 30-35 minutes, or until the oatmeal is set and golden brown around the edges. A toothpick inserted in the center should come out clean.
07 - Allow the baked oatmeal to cool for 5-10 minutes before slicing. Cut into 6 equal portions. Top each serving with Greek yogurt, fresh banana slices, chopped nuts if desired, and a drizzle of maple syrup or honey.