Adjust the oven rack to the lower third position and preheat the oven to 350°F (177°C). Spray a 9×9-inch square pan, or an 11×7-inch rectangle pan with nonstick spray. Any similar-size pan that holds about 3 quarts works, though an 8×8-inch square pan would be too small. The pictured dish I’m using is this white casserole dish. See recipe Note for 9×13-inch pan.

In a large bowl, whisk together the milk, eggs, maple syrup, melted butter, mashed banana, and vanilla until combined. Add in the oats, baking powder, cinnamon, and salt, and whisk to combine. Fold in the chopped walnuts, if using. Pour into the prepared baking pan.

Bake for 30–35 minutes or until the center appears *almost* set, which gives us a soft baked oatmeal as pictured above. For drier and more solid baked oatmeal, bake until the center has set.

Cool for 5 minutes before serving. Spoon or slice and serve with optional toppings, if using.

Cover leftovers tightly and refrigerate for up to 1 week.