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Preheat the oven to 375°F. Grease 12 standard or 40 mini muffin cups with oil or line with paper liners. Set aside.
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In a medium bowl, whisk together the whole-wheat pastry flour, oat flour, flax meal, baking powder, baking soda, and salt. Set aside.
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In another medium bowl, whisk together the buttermilk, egg, orange juice, orange zest, and granulated sugar until smooth. Slowly drizzle in the butter, whisking constantly, until it is fully incorporated.
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Add the liquid ingredients to the dry ingredients, using a rubber spatula to gently stir them together until just combined. Add the cranberries, stirring with as few strokes as possible. If you stir the batter too much, it will result in tougher muffins (we want tough kids, not tough muffins!).
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Divide the batter evenly among the muffin cups, filling them about 3/4 full.
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To make the streusel topping, in a small bowl, stir together the oats, brown sugar, walnuts, cinnamon, and salt. Break the butter into little bits and use your hands to work it into the oat/sugar mixture. Sprinkle it over the muffins, dividing it evenly.
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Bake until a toothpick inserted into the center comes out clean, 12 to 15 minutes for mini muffins, 20 to 25 minutes for regular muffins.
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Leave muffins in the tins until cool enough to handle, then use a little knife to dislodge them and continue to cool on the counter. Store in an airtight container for up to 3 days in the pantry or 1 month in the freezer.
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Note: If you don’t have oat flour in your pantry, you can make it by pulverizing 1 scant cup of rolled or quick oats in a blender. It should yield 1 cup oat flour.