Blend bananas and milk until smooth. Add spinach and blend on a high setting until most of the spinach has been broken down into small pieces. Add the frozen berries and blend until the smoothie mixture is all one color.
Add the bran and sweetener; blend until desired consistency. Add ice cubes and blend again until smooth (optional - I usually don't).
Notes
I like to blend it well for several minutes so it's mostly smooth, but there is still some crunch and texture from the bran.
Recipe by Make Mine Gluten Free at https://www.makemineglutenfree.com/berry-green-smoothie/