Preheat oven to 350°F. Grease a 9x13-inch baking dish or line with parchment.
In a large bowl, whisk milk, eggs, maple syrup, vanilla, and melted butter until smooth.
Stir in the mashed banana until evenly combined.
Add oats, baking powder, cinnamon, and salt. Mix until the oats are fully moistened.
Fold in blueberries and walnuts if using.
Pour the batter into the prepared dish and smooth the top.
Bake for 35 minutes, or until set in the center and lightly golden at the edges.
Cool for 10 minutes. Slice into 8 squares and serve warm or at room temperature.
Notes
Storage: Refrigerate in an airtight container for up to 4 days or freeze for up to 3 months. Reheat in the microwave or a 300°F oven until warmed through. Dairy-free: use almond milk and coconut oil. Nut-free: omit walnuts. For extra protein, stir in 0.25 cup ground flaxseed or add a scoop of protein powder with the dry ingredients.