Cook's Note: To toast the walnuts, put them in the oven at 375 degrees F until nicely toasted, about 10 minutes.
1. Preheat the oven to 375 degrees F. Adjust the oven rack to the center. Line a 12-cup muffin tin with your favorite liners and lightly grease with cooking spray.
2. Finely chop the walnuts in a food processor. Set aside 3 tablespoons for garnish.
3. Combine the remaining ground walnuts, amaranth flour, brown rice flour, tapioca flour, baking powder, cinnamon, salt and baking soda in a large bowl. Add the sugar and work in with your hands.
4. Whisk together the bananas, milk, oil, vanilla and egg yolks in a separate medium bowl.
5. Whip the egg whites on medium-high speed in the bowl of a large stand mixer fitted with a whisk attachment (or in a large bowl of using a hand mixer) until soft peaks form, 2 to 3 minutes.
6. Fold the banana mixture into the nut-flour mixture until combined (don't fret if your batter has lumps, this means it isn't over-worked). Fold one third of the whipped egg whites into the batter until completely incorporated, then gently fold in the remaining egg whites until just combined (it's alright if the batter is a little streaky).
7. Fill each liner about three-quarters full. Sprinkle with the reserved ground walnuts and turbinado sugar, if using.
8. Bake until the muffins are golden brown and nicely cracked on the top, and a toothpick inserted in the muffins comes out clean, 28 to 30 minutes, rotating halfway through baking. Let cool in the hot muffin tin 3 minutes, then transfer to a cooling rack to cool completely.