Preheat oven to 400°F (204°C). line with muffin tin with muffin liners. Set aside.
Whisk the flour, oatmeal mixture, baking soda, baking powder, salt, and cinnamon mix together.
In a large bowl, add oil, egg, brown sugar, milk, apple sauce and mix well to this add sourdough starter until all wet ingredients are combined. Then add grated apple and apple pieces and mix once again.
To the wet ingredients add dry ingredients and mix until *just* combined. Do not over-mix. Batter will be thick and a little lumpy.
In a small bowl make topping, add ¼ cup oatmeal mixture and rest of ingredients for the topping and mix well and set aside.
Divide batter among prepared muffin pan, filling all the way to the top. Then top with 1 tsp of topping.
Bake at 400°F for 10 minutes, then, keeping the muffins in the oven, reduce the oven temperature to 350°F (177°C) and continue to bake for 15 minutes or until centers are set and toothpick comes out clean.
Enjoy with cup of coffee of milk.