Preheat the Oven / Toast the Walnuts: Preheat the oven to 375F (190C). Toast the walnuts on a small sheet pan while the oven is preheating, about 15-17 minutes. Keep an eye on them starting at 14 minutes as they go from not toasty to over toasty fast!
Mix the Wet Ingredients: While the oven is preheating and walnuts are toasting, in a large mixing bowl, mash the bananas, pour in the coconut oil, syrup, nut milk, apple cider vinegar and vanilla extract. Whisk until smooth.
Add the Dry Ingredients: To the wet ingredients add the oat flour, almond flour, tapioca flour, flax meal, baking powder, baking soda and salt. Whisk all the ingredients until there are no flour bits left and the mixture is smooth. Set aside and rest the batter while the walnuts cool.
Remove the walnuts from the oven. Transfer the walnuts out of the pan and onto a cutting board. Spread the walnuts out to cool slightly. Rough chop. Set about 2 Tbs aside to be sprinkled on top of the muffins prior to baking. Then, fold the walnuts into the batter just before scooping.
Scoop / Pan the Muffin Batter: Line a standard muffin pan with 12 tulip style muffin cups (DIY your own). Scoop about 1/3 C (85g) of batter into each cup. An overfilled large cookie scoop works well too! Sprinkle a bit of chopped walnuts and a few grains of whole rolled oats on the top of each muffin if desired.
Bake the Muffins: On the center rack in the oven, bake for about 25-30 minutes OR until the muffins are golden, a toothpick inserted in the center of a muffin comes out clean and the muffins spring back under gentle pressure. Remove from the oven, and pan, transferring the muffins to a cooling rack. Allow to cool completely before unwrapping and enjoying.
Store the Muffins: After completely cooled, store muffins covered at room temperature for up to three days, or freeze in a freezer container for up to two weeks. Thaw at room temperature. To rewarm the muffins, warm in a 350F oven for about 10 minutes.