Preheat the oven to 375°F (190°C).
Spray an 8-inch square baking dish with cooking spray.
In a large bowl, combine the oats, baking powder, cinnamon, and salt.
Stir in the milk, maple syrup, egg (or flaxseed mixture), melted coconut oil, vanilla, and mashed banana until fully mixed.
Gently fold in 1 cup of strawberries, half of the banana slices, and ⅛ cup of chocolate chips.
Transfer the oatmeal mixture to the prepared baking dish.
Scatter the remaining strawberries, banana slices, and chocolate chips evenly on top.
Bake for 50–60 minutes, or until the center is mostly set. It may look slightly moist due to the strawberries, which is normal. The edges and top should turn golden.
Let the baked oatmeal cool for a few minutes before slicing. Serve warm with extra fresh strawberries, banana slices, and a splash of milk.