Preheat oven to 400°. Line a 12-cup muffin tin or lightly grease with coconut oil.
Pulse 1/3 cup oats in food processor until it's the consistency of flour.
Combine the processed oat flour, 1/2 cup whole oats, baking powder, baking soda, salt and cinnamon in a medium bowl.
In a large bowl, mix eggs, honey and mashed bananas. Add dry ingredients to banana mixture; stir until combined. Fold in apples and walnuts.
Fill muffin tins evenly, and garnish with additional apple slices and walnuts, if desired. Bake for 18 to 20 minutes, or until baked through. Remove from muffin tin, and cool on wire rack.