Add the oats and milk to a pan and cook on a low heat for a few minutes, stirring occasionally with a wooden spoon.
In the meantime, cut the apple into quarters, remove the core and chop into small pieces. Add the chopped apple to the oat mix and continue to cook for a few minutes until the apple softens.
Stir in the protein powder and cinnamon and take off the heat.
Serve warm, topped with chopped mixed nuts.