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Preheat oven to 350F. Grease and flour a 12 cup muffin tray or line with muffin cups.
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Blend together bananas, milk, and olive oil until smooth using a blender, food processor or an immersion blender.
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In a separate large bowl mix together flours, 1/3 cup sugar, baking powder, baking soda, salt, cinnamon, and nutmeg with a wooden spoon.
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Add the blended ingredients to the bowl and mix again mix with a wooden spoon.
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Stir in diced apple and finely chopped walnuts. Evenly distribute muffin dough into muffin cups.
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Place 2 tablespoons sugar, walnuts, oats, and coconut oil in a food processor to make streusel topping. Pulse until there are no visible large walnut chunks. Crumble streusel topping over muffins.
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Bake in oven for 15 to 18 minutes. Remove from oven and allow to cool completely on wire rack.