Add the milk, frozen banana slices, oats, peanut butter, flax seeds (if using) and spices to a high-speed blender.
Blend on high or use the smoothie program of your blender. I blend it for 30 seconds to 1 minute to make sure the oats are as finely ground as possible.
Video
Notes
Leave out the ground ginger, nutmeg and cardamom for a plain cinnamon banana smoothie.
For a spiced chocolate smoothie, add 2 heaped teaspoons of unsweetened cocoa powder.
If you don't have frozen bananas on hand, use fresh ones and add 2-3 ice cubes. The smoothie won't be as thick and creamy, but the taste will be the same.