Preheat your oven to 375°F (190°C). Line a baking sheet with parchment paper to prevent sticking.
In a large mixing bowl, whisk together the buttermilk, egg, brown sugar, vanilla extract, and vegetable oil until well combined.
In another bowl, combine the quick oats, all-purpose flour, baking soda, salt, baking powder, and cinnamon. Mix well to ensure that the baking soda and baking powder are evenly distributed.
Gradually add the dry ingredients to the wet mixture, stirring gently until just combined. Be careful not to overmix, as this can lead to tough muffin tops.
Gently fold in the blueberries and chopped walnuts, ensuring even distribution throughout the batter.
Using a spoon or cookie scoop, drop generous amounts of the batter onto the prepared baking sheet, making sure to leave space between each muffin top. Bake in the preheated oven for 15-18 minutes or until the tops are golden brown and a toothpick inserted in the center comes out clean.
Allow the muffin tops to cool on the baking sheet for a few minutes before transferring them to a wire rack. Once they are cool enough to handle, enjoy them warm, or store them for later!
