Peel the bananas and chop them. If you have time, pop them in the freezer for 4-8 hours. This will give you a cooler, creamier smoothie.
When you’re ready to make your smoothie, tip the bananas into the blender. Add the kefir and walnuts. Grate in ¼ of the nutmeg. Blend till smooth with a cupful of ice. Taste and add a little more nutmeg if you like. Serve straight away.