Preheat your oven to 375°F (190°C). Line a muffin tin with paper liners or grease it lightly.

In a medium bowl, whisk together the all-purpose flour, instant oats baking powder, baking soda, salt, and cinnamon. Set it aside.

In a large bowl, add egg, oil, sourdough starter, vanilla extract and mix well then add brown sugar and granulated sugar mix until it is light and fluffy.

Gradually add the dry ingredients to dry ingredients and then fold in mashed banana and grated carrot mixing just until combined . Be careful not to overmix the batter.

Then add toasted walnuts and combine once again. Reserve 2 tablespoon crushed walnut aside

Spoon the batter into the prepared muffin tin, filling each cup with about two-thirds full. To this top with crushed walnuts .

Bake in the preheated oven for about 18-20 minutes or until a toothpick inserted into the center of a muffin comes out clean.

Remove the muffins from the oven and let them cool in the tin for few minutes before transferring them to a wire rack to cool completely.