Preheat the slow cooker if necessary; see the manufacturer's instructions. Pour the boiling water and milk into the slow cooker pot, then stir in the oats.
Cover with the lid and cook on low for 1 hour for 'runny' porridge or 2 hours for 'thick' porridge.
Spoon into bowls, slice the bananas and divide among the bowls. Mix together the sugar and cinnamon and sprinkle over the top.