Line the base and sides of a 30 × 25 cm (12 × 10 inch) tray with freezer film. Quarter the apples and remove the cores. Place in a saucepan, cover with water, add the lemon juice and bring to the boil. Reduce the heat to low and simmer for 15 minutes or until very soft. Strain, discarding solids. Return the liquid to the pan, place over medium heat and cook until reduced to 250 ml (9 fl oz/1 cup). Add the tonka bean, xanthan gum, kappa and iota and bring to the boil for 1 minute. Spread evenly over the lined tray and refrigerate for 1 hour or until set.
This can be made up to 1 day in advance. When ready to serve, cut the jelly into six 15 × 3 cm (6 × 1¼ inch) strips. Set aside.