In a large mixing bowl, combine the whole wheat flour, rolled oats, baking powder, baking soda, cinnamon and salt and mix well until evenly combined. Set aside.
If you're planning on using the garnish, make it now: mix the rolled oats, chopped walnuts, melted ghee, honey and cinnamon together in a small bowl; reserve.
In a large bowl, mash the bananas with a fork until they are reduced to a smooth puree.
Add the eggs and mix them in with your fork until very well combined, then add the buttermilk and melted ghee and mix until fully incorporated.
Throw in the dry ingredients and mix until just barely incorporated, then fold in the chopped walnuts and again, stir until just incorporated, no more. Delicately scrape the bowl with a rubber spatula to make sure you have all the dry ingredients mixed in.
Divide the batter between 12 large muffin cups. The batter will need to be mounded higher than the paper cups, so try and shape the tops into nice rounded domes. The use of a large spring-loaded ice cream scoop is strongly recommended. Alternately, you could also divide the batter into 18 to 24 smaller muffins. Of course, you'd need to adjust cooking time accordingly.
Delicately spoon the reserved garnish (if using) over the tops of the muffins.
Bake in 400°F oven for 20 minutes, or until a toothpick inserted in the center of a muffin comes out clean. Let the muffins cool for a few minutes in the pan then remove to a wire rack to cool completely.
Store in a cool dry place in an airtight container for about a week.