Preheat oven to 350°F. Grease a 12 cup muffin pan with cooking spray of choice. Set aside.
Place all of the muffin ingredients, except for the pecans, in the order listed, in a high powered blender. If you do not have a high powered blender you can use oat flour instead of rolled oats and mix the ingredients together with a hand mixer. Blend for 60 seconds or until a smooth batter forms. Stir in the pecans or walnuts.
Fill the muffin cups 3/4ths full with the batter, dividing it evenly amongst the cups. In a small bowl combine all the streusel ingredients. Sprinkle over top of each batter filled muffin cup.
Place the pan inside the preheated oven and bake for 16 minutes or until a toothpick is inserted and comes out clean. Let cool for 5 minutes before running a butterknife along the sides of the muffin cups, carefully remove each muffin and serve warm.
The streusel topping is completely optional, it elevates the recipe, but they're great without it too!