If baking now, preheat oven to 375 degrees F (190 C). In a large bowl whisk the starch into the milk. Add two of the bananas, and mash with a fork. Add the peanut butter, maple syrup, vanilla, cocoa, cinnamon, and salt, and whisk to combine.
Peel and slice one banana, and add to the bowl along with the oats. Stir to combine. Pour into a 9-inch square baking dish, and smooth the top.
If baking now, peel and slice the remaining banana, and arrange slices on top. Crumble the walnuts, and sprinkle on top, along with an extra dash of cinnamon, if desired. If assembling in the eveningand baking in the morning, don't add the banana slices or walnuts yet. Cover the dish and refrigerate overnight. In the morning, preheat oven to 375 degrees F (190 C), and arrange sliced banana and nuts on top.
Bake for 30 to 35 minutes or until hot in the center. Let stand 10 minutes before serving.
Store leftover baked oatmeal in the refrigerator for up to 5 days.