Put millet and oat milk in a small saucepan and bring to the boil briefly. Then, half covered, cook over a low heat for about 35-40 minutes. Stir occasionally, possibly adding a little more oat milk.
Add cinnamon, sea salt, cashew butter and agave syrup to the millet, possibly add a little oat milk to make the mixture creamier. Let cool down briefly. Cut and core the apple and add with the cranberries.