Preheat oven to 350°F. Lightly coat an 8x11 baking dish with nonstick cooking spray.
Use a fork to mash the bananas in the baking dish. Add the oats, followed by the chia seeds and milk. Stir to combine. Add the frozen mixed berries on top. Then sprinkle it with cinnamon and lemon zest.
Bake in the preheated oven until the liquid is absorbed, about 25 minutes.
Drizzle maple syrup on top before serving, if desired.
Video
Notes
Storage: Store any leftover oatmeal in the baking dish tightly covered with plastic wrap or in an airtight container for up to 4 days. Reheat in the microwave or in the oven until warmed through.