Preheat oven to 350°F. Line a 12-cup muffin pan with muffin liners. Set aside.
Peel bananas and add to a large mixing bowl. Using a dough blender (recommended) or sturdy fork/whisk mash bananas until smooth. Add the coconut oil, maple syrup, flax egg and vanilla. Whisk until well incorporated.
Add oat flour, almond meal, baking powder, baking soda and salt. Whisk together until just incorporated, making sure no flour patches remain. Fold in chopped walnuts.
Using a muffin batter scoop, scoop and drop batter evenly into muffin cups. Fill each cup all the way to the top for a smaller quantity of big, round-topped muffins, and ¾ of the way for a larger quantity of smaller muffins. Optionally, press walnuts into the muffin tops.
Bake for 20-30 minutes. Mine took 25 minutes for large, high-topped muffins. Place muffin pan onto a cooling rack for 30 minutes. Then, gently transfer muffins directly onto cooling rack for 1 hour to cool completely. Enjoy. Storing instructions below.
A Note on Homemade Oat Flour: *If using homemade oat flour, make sure to blend the flour until very fine and sift the flour to remove coarse bits of oats that could create less than ideal results. Store until needed, then measure the amount required for the recipe.Storage Notes: Store in an airtight container for up to 1 week.Freezing Instructions: These muffins freeze well. Store in a freezer-friendly, airtight container in the freezer for up to 1 month. Allow to thaw at room temperature for 30 minutes, or pop in the microwave for 30 seconds or more.