Peel, core and roughly chop the apple and put into a saucepan with the small sprig of rosemary, the teaspoon of sugar, the lemon zest and juice, and butter. Cover the pan and cook on a low heat for 4-8 minutes until the apple is soft. How long this takes really depends on the variety of apple you're using. Coxes cook the fastest, and are good here.Leave to cool, and fish out the rosemary sprig when it is cold.Preheat the oven to gas mark 3/170°C/150°C Fan/325ºF. Line a 450g / 1lb loaf tin with a loaf liner, or butter and line the bottom with baking parchment.Put the cooled apple into a food processor and blitz to a pulp. Then add the butter, 150g / ¾ cup of sugar, eggs, flour and baking powder and process to a smooth batter.Spoon and scrape into the loaf tin and smooth the top. Sprinkle the surface with the remaining tablespoon of sugar and then lay the long sprig of rosemary along the centre of the cake. On baking, the rosemary sheds its oil to leave a scented path down the middle of the cake.Bake the cake for 50 minutes or until a cake tester comes out clean, then leave to cool on a rack. Slip the paper-lined cake out of the tin once it is cool.