In a large bowl, stir together the oats and milk. Cover and let soak in the fridge for 5-6 hours, or until the oats have fully absorbed the milk.
Preheat oven to 425F and line a 12-cup muffin pan with parchment liners.
In a medium-sized bowl, whisk together the flour, baking powder, baking soda, cinnamon and salt.
In a large bowl, whisk together the soaked oats, mashed bananas, egg, oil, honey, coconut sugar and vanilla.
Add the dry ingredients to the wet mixture and stir together until almost combined.
Add the walnuts and stir until there are no streaks of dry ingredients left. (Be careful not to overmix).
Divide the batter evenly into the prepared muffin pan, filling each almost to the top.
Bake at 425F for 5 minutes, then reduce the temperature to 350F and bake for another 12-14 minutes, or until a toothpick inserted into a muffin comes out clean.
Let cool in the pan for 5 minutes before transferring to a wire rack to cool completely.
Notes
For a nut-free version, substitute dairy milk for almond milk, and replace the walnuts with pumpkin seeds, raisins or chocolate chips.
For a gluten-free version, substitute the whole wheat flour for gluten-free 1-to-1 baking flour and make sure your oats are certified gluten-free.
For a vegan version, replace the honey with maple syrup and use a flax egg instead of an egg.
If you're pressed for time, you can use old fashioned rolled oats and soak them for 30-60 minutes. They won't create the same nutty texture, but they still work well in this recipe.
Dark colored muffin pans tend to brown the muffins faster, especially on the bottom. So, if you're using a dark pan, make sure to use muffin liners and keep an eye on the muffins as they near the end of their bake time.