Preheat the oven to 350 degrees F. Line a 12-muffin tin with paper liners and spray with cooking spray.
Whisk together the flour, bran flakes, 1/3 cup of the chopped walnuts, baking powder, cinnamon, 1/2 teaspoon salt, baking soda and nutmeg in a large bowl. Whisk together the brown sugar, oil, sour cream, bananas, eggs and vanilla in a medium bowl. Add the wet mixture into the dry mixture and gently fold until just incorporated (it's OK if there are lumps in the batter).
Evenly divide the batter among the muffin cups and sprinkle each with some of the remaining 1 tablespoon walnuts and turbinado sugar if using. Bake until the muffins are golden brown, bounce back when pressed and a toothpick inserted in the center comes out clean, 20 to 22 minutes. Let cool in the pan for a few minutes, then remove and cool completely on a rack. Store in an airtight container at room temperature for up to 3 days.
To toast the walnuts, spread out the nuts on a baking sheet and toast in a 350 degrees F oven until golden, about 7 minutes. Check frequently, as they can burn quickly.
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