Fold in the rhubarb, apple, walnuts, baking powder, flour and oat bran, then spoon the mixture into the cases. Mix together the demerara sugar, cinnamon and nutmeg and sprinkle over the muffins.

In a separate bowl, stir together the caster sugar and diced rhubarb. Let it sit for a moment ... oven and bake for 15-18 minutes, or until the muffins are golden brown and a cocktail stick ...