Preheat oven to 375°F/190°C. Lightly grease the cups of a muffin tray with cooking oil or butter.
In a large bowl, sift in the flour, polenta, baking powder, salt and bicarb of soda. Stir in the apple chunks, cheese and thyme.
In a small bowl, lightly whisk together the egg, milk and mustard.
Pour the milk mixture into the flour mixture and stir to combine.
Spoon the batter into the muffin cups ¾ full. Sprinkle the remaining cheddar over the batter.
Bake for 15-20 minutes or until the muffins have risen and turned a light golden colour. Sprinkle the top with the remaining thyme leaves onto the melted cheese.
Allow to cool in the tins for 5 minutes, then remove and cool completely on a wire rack.