Preheat oven to 375 degrees. In a medium bowl, mix together the oats, baking powder, cinnamon, and salt.
In a large measuring cup, whisk together the milk, honey, eggs, vanilla. Add liquid mixture to dry mixture in the bowl and stir to just combine. Drizzle in butter or coconut oil while stirring, then add mashed banana, blueberries, and nuts.
Grease an 8x8 or 9x9 baking dish with butter or cooking spray. Pour oat mixture into pan and bake for 28-33 minutes or until the baked oatmeal is set and a toothpick inserted in the center comes out mostly clean.
Let cool for 10 minutes in pan and serve, or refrigerate in a storage container for up to 5 days, reheating in microwave before serving.
To double this recipe, bake in a 13x9 inch dish for 43-48 minutes at 375 degrees and cover with foil during last 15 minutes of baking to prevent over-browning.*I've started omitting the honey altogether to reduce the sugar, and it still tastes great (if you're used to less sweet baked goods)!