Preheat oven to 375°F (190°C). Line or grease a 12-cup muffin tin.
In a large bowl, whisk flour, oats, baking powder, baking soda, salt, cinnamon, nutmeg.
In another bowl, mash bananas with a fork or masher.
Add melted coconut oil, almond milk, maple syrup, vanilla, and flaxseed to bananas. Mix well and rest 2 minutes.
Pour wet into dry, gently fold until just combined. Batter should be slightly lumpy.
Fold in ¾ of walnuts. Don’t overmix.
Divide batter evenly among muffin cups. Top with remaining walnuts.
Bake 18–22 minutes, until a toothpick comes out with a few moist crumbs.
Cool muffins on a wire rack before serving.