Set an oven rack on the center shelf and preheat the oven to 375 degrees F. Grease an 8x8-inch pan with cooking spray or butter.
In a medium bowl, combine the oats, half the almonds, baking powder, cinnamon, and salt.
In a separate bowl, combine the milk, maple syrup, egg, butter, and vanilla.
Arrange the bananas in a single layer in the bottom of a greased 8x8 baking dish. Top with 2/3 of the berries, then top evenly with the oat mixture. Slowly drizzle with the milk mixture, gently shaking the pan to incorporate. Top with the remaining berries and nuts.
Bake the oatmeal for 35-45 minutes until golden on top and set. Cool for a few minutes before serving. You can optionally serve the oatmeal with a bit more melted butter drizzled on top.