Special equipment: Waffle iron (preferably Belgian style)
Preheat a waffle iron to medium-high. Preheat the oven to 200 degrees F (to keep cooked waffles warm). Whisk together the flour, sugar, walnuts, baking powder, baking soda, cinnamon, nutmeg and salt in a large bowl. Whisk together the banana, oil, sour cream, vanilla and eggs in another bowl. Fold the banana mixture into the flour mixture until just combined (it's ok if there are lumps).
Lightly brush the top and bottom of the waffle iron with oil. Fill each section about three-quarters of the way full (there should still be some waffle iron showing). Close and cook until the waffles are golden brown, 4 to 6 minutes. Keep the waffles warm in the oven or covered with foil on a plate while you cook the remaining batter.
Spread with butter and dust with confectioners' sugar if you'd like.
Freeze the waffles in resealable plastic bags for up to 1 month. Reheat in a toaster oven or 300 degree F oven until warmed through and crisp.
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