Add all the ingredients (other than the walnuts, if using) to a high-speed blender, and blend until just smooth. Do not over-blend. You may need to stop blending to scrape the sides once or twice.
Grease or line a 12-cup muffin pan.
Spoon or pour 3-4 heaping tablespoons of batter into each cup. Sprinkle with walnuts, if using.
Bake the muffins for 20-25 minutes, or until the tops are slightly browned and a toothpick comes out clean when inserted into the center of a muffin.
Allow to cool in the pan for 5 minutes, then cool the rest of the way on a cooling rack.
Notes
If you are gluten-free be sure to buy oats/oat-flour that are specifically labeled gluten-free.