Mix the flour, baking powder, baking soda, salt, cinnamon, and ½ cup rolled oats in a bowl.
In a large bowl, mash bananas with a fork. Add sugar, egg, melted butter, and vanilla extract and mix well. Next, add sourdough starter and mix well.
Fold the flour and oatmeal mixture into the banana and sourdough mixture. Make sure not to over-mix the batter. If you see some dry bits, that is okay.
Finally, fold in chopped pecans.
Add equal amounts of batter to each muffin well. I like to use an ice cream scoop for this.
Top with nut crunch topping.
Bake it in pre-heated oven at 375 °F for 18-20 minutes or until a toothpick inserted comes clean.
Cool the muffin completely in the wire rack.