1.Preheat oven to 180°C (160°C fan-forced). Grease a 19cm x 30cm lamington pan; line base with baking paper, extending paper 5cm over two long sides.
2.Sift dry ingredients into large bowl; stir in apple, eggs and oil. Pour mixture into pan and bake about 40 minutes. Stand slice in pan 10 minutes before turning, top-side up, onto wire rack to cool.
3.To make maple syrup icing, sift icing sugar into small bowl; stir in the hot water and maple syrup.
4.Spread slice with icing and sprinkle with chopped walnuts.