Preheat the oven to 350F. Line a standard muffin pan with parchment muffin cups.
In a large bowl, add the banana and mash it up (you should have about 1 cup). Add the yogurt, oil, maple, eggs, and vanilla. Whisk until combined.
Add the oats, flour, cinnamon, baking soda, and salt and mix until just combined. Stir in the raspberries and half of the nuts.
Divide between the muffin cups, filling to the brim (about a heaping ⅓ cup per muffin). Top with remaining walnuts and gently press them into the batter. Bake for 25-30 minutes or until they feel firm on top and a knife inserted in the center comes out clean.
Let the muffins cool down to help the muffin liners peel off more easily if they are sticking.