Preheat your oven to 350°F (175°C).
In a mixing bowl, combine the oats, baking powder, cinnamon, and salt. Stir until well mixed.
In another bowl, whisk together the milk, egg, honey or maple syrup, and vanilla extract until fully combined.
Add the sliced bananas and blueberries to the wet mixture, gently folding them in.
Pour the wet ingredients into the bowl with the dry ingredients. Stir until just combined.
Fold in the chopped walnuts or pecans.
Grease your 9x9-inch baking dish and pour the oatmeal mixture into it. Spread it out evenly.
Place the baking dish in the preheated oven and bake for 30-35 minutes.
Once baked, let it cool for a few minutes. Cut into squares and serve warm.
