Combine the flour, sea salt, and granulated sugar into a small to medium sized mixing bowl.
MAKE TOPPING MIXTURE
Take out about 88 g (½ Cup) of the mixture and put into a small to medium-sized bowl for the beginning of your topping mixture.
FINISH DRY MIXTURE
Return to your dry mix and add the baking soda, baking powder, quick oats. Whisk until well combined and then add the chopped nuts and toss until all nuts are well incoporated.
START THE WET MIXTURE
In a separate, larger bowl, cream together the melted (then cooled) butter and brown sugar.
Add the mashed bananas, sourdough starter and then whisk together until there are only small lumps.
COMBINE MIXTURES
Be careful not to over mix the dry ingredients.
Gradually add the dry mixture to the wet mixture. Ideally, it should be more of a folding motion rather than mixing or stirring one. (I like to add the dry mix in 3 rounds).
BAKE & SERVE
Pour the batter into the 12-cup muffin pans which are either lined with paper liners or well-greased.
For the topping, I like to add 1 tsp heap of the Topping mixture to each muffin, go back over with a fork to make sure all of the top are covered. I even might "puncture" the batter and mix the topping in a little bit to add more flavor.
Then with the remaining topping, I'll take a 1/2 tsp and add to the tops again, just gently evening it out across the top.
Place the pan in the middle of the rack, in the upper half of the oven and bake for 20-25 minutes.
You'll know the muffins are ready when you poke a toothpick into the middle of a middle muffin, all the way down and it comes out clean.