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 baking powder, cinnamon, nutmeg and sugar, and set aside. Add nondairy 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 375F. 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 you can't stir it easily, thin it out with a little nondairy milk, but you shouldn't need to. It should be thick like oatmeal or heavy mashed potatoes. Spoon into 12 cups, top with cinnamon (and brown sugar if you like) and bake 10-15 minutes, or until firm to the touch and a toothpick inserted comes out clean.
Note: If you have a rather ripe (very sweet) 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.