Preheat the oven to 110°C/fan90°C/gas 1/4. Line 2 large baking trays with non-stick baking paper.
Put the sugar, lemon juice and 90ml water in a pan over a low heat. Stir to dissolve. Bring to the boil and cook for 3-4 minutes until just syrupy. Set aside.
Slice the apple finely, discarding the ends. Dip in the syrup with tongs, let the excess drip off and put on the trays in 1 layer. Repeat with a second apple. Bake for 4 hours, turning halfway, until dried. Transfer to a rack to cool and crisp.