Pre-heat oven to 400 degrees and spray a muffin pan with cooking spray or line with baking cups.
In a large bowl, mix together wet ingredients: mashed bananas through yogurt; set aside.
In a medium bowl, whisk oat flour (see note below), oats, brown sugar, baking soda, baking powder, cinnamon, nutmeg and salt.
Slowly combine the dry mixture with the wet mixture and stir until incorporated. Add in ¼ cup of the walnuts and stir again.
Divide batter evenly into muffin tins and sprinkle each with teaspoon or so of remaining chopped walnuts. Bake for 18-20 minutes or until toothpick comes out clean.
Let cool in pan for two minutes then remove to cool completely on a cooling rack.
Notes
To make oat flour, simply pulse 2 cups old fashioned oats in a food processor to create a fine meal. This should only take about 60 seconds!