STREUSEL: I prepare this in a mini food processor. Add flour, brown sugar, cinnamon, and nutmeg to bowl of processor/chopper and pulse a few times to blend. Add the butter a fourth at the time, pulsing between additions. Doing it this way will keep you from having large lumps of butter. With the final fourth of butter, pulse the ingredients until it looks like wet sand with small clumps. Add the oatmeal and pulse just until the oatmeal is distributed. Don't over do or else the oatmeal will be more like oat flour rather than the nice chewy pieces we're looking for! If desired, stir in the pecans or walnuts by hand. Refrigerate until needed so the butter stays nice and cold.
MUFFINS: In a large bowl, combine flour, sugar, baking soda, salt, cinnamon, and nutmeg.
In another bowl, mash bananas well, leaving a few very small chunks.
Add egg, oil, and vanilla to the bananas and stir well until combined.
Stir banana mixture into the dry ingredients, just until moistened.
Measure 1/4 cupful of batter into 12 well greased or paper-lined muffin cups.
Top each muffin with two heaping tablespoons of the streusel. You may have extra, but we like a lot of topping, so I manage to use it all :-)
Bake at 375 degrees for 17-22 minutes, or until a toothpick inserted near the center comes out clean. It took 17 minutes in my oven.
Allow to cool in pan 5 minutes.
Remove the muffins from the pan to a wire rack to cool completely.
NOTE: The streusel can be made using a pastry blender or two knives.