Transfer rolled or instant oats to a blender and whiz for a few seconds until flour forms. Carefully spoon out 2 cups of oat flour and transfer to a mixing bowl.Whisk in a baking powder, cinnamon, nutmeg and sugar, and set aside. Add non-dairy milk and banana to the blender and whiz until well combined. Pour into oat flour mixture, add raisins and stir to combine. It should be rather soupy. Add a handful about (1/3 cup) rolled oats and stir. If it's still runny,add more oats. I usually add about 1/2 cup total. The batter should be thick like oatmeal (ha-ha!) but not dry. Set batter aside and preheat oven to 375 F. Grease a muffin pan and spoon batter into the pan. The batter will thicken and get a little sticky/tacky as it sets. If it's so thick like oatmeal or heavy mashed potatoes. Spoon into 12 cups, top with cinnamon (add brown sugar if you like) and bake 10-15 minutes, or until firm to the touch and a toothpick inserted comes out clean.
If you have a rather ripe banana, you probably don't even need the sugar, especially if you're used to eating unsweetened oatmeal. Or you could add a few dates in with the nondairy milk and banana for a naturally sweet alternative.