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 smooth and well combined.
03 - In a separate medium bowl, combine the rolled oats, baking powder, cinnamon, nutmeg, and salt. Whisk to evenly distribute spices.
04 - Pour the dry ingredients into the wet mixture. Stir with a spatula until just combined, being careful not to overmix.
05 - Pour the batter into the prepared baking dish and spread evenly with the spatula.
06 - Bake for 30 to 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 - Let the baked oatmeal cool for 5 to 10 minutes before slicing. Top each portion with Greek yogurt, fresh banana slices, nuts if desired, and a drizzle of maple syrup or honey.