1 Place the sugar and 500ml (18fl oz) water in a large, heavy-based saucepan on a medium heat and bring to the boil, stirring until the sugar dissolves. Reduce the heat to low and simmer for about 25 minutes without stirring, until a sugar thermometer dipped into the mixture reads 125°C (257°F). Alternatively, check that the hard-ball stage has been reached.
2 In the meantime, stir the gelatine, cornflour and cream of tartar together in another large pan. Measure out another 500ml (18fl oz) water and gradually whisk it into the mixture, beating well after each addition, to form a smooth paste. Be sure to break up any lumps as the gelatine may cause the mixture to stick together a little. Place the pan on a medium heat and simmer, whisking all the time, for 3–5 minutes or until the mixture thickens. Remove from the heat.
3 Stir the lemon juice into the sugar syrup as soon as it reaches the correct temperature (standing back as it will sizzle up a little). Working carefully, gradually pour the sugar syrup into the gelatine and cornflour mixture, whisking constantly.
4 Place the pan on a low heat and simmer the mixture gently for about 1 hour until it reaches 110°C (230°F) in temperature or the thread stage. Stir frequently to prevent the mixture sticking to the pan, particularly towards the end of cooking. Grease the cake tin with the sunflower oil.
5 As soon as the mixture reaches the correct temperature (it should be a deep golden colour), add the rosewater, pistachios and food colouring and stir until well combined. Pour the mixture into the prepared tin, spreading out evenly. Set aside to cool to room temperature before placing in the fridge for 4 hours or overnight until firm.
6 Spoon the icing sugar into a fine sieve and dust half of it onto a clean work surface. Turn the Turkish delight out of the tin and use a lightly oiled knife to cut it into about 48 pieces. Dust with the remaining icing sugar and toss the pieces about to coat generously. Store in an airtight container in a cool, dry place.