Heat the slow cooker if necessary. Put the barley and 750ml boiling water into the slow cooker. Peel and core the apples so you have a hole the size of a pound coin in each one. Cut each apple in half.
Stand the apples skin side down on the barley. Mix the cinnamon, nutmeg and orange zest, and sprinkle them over the apples.
Cook on Low for 2 hours. Serve with natural yogurt.