Preheat the oven to 180C or 356F.
Peel the jicama and slice into small, bite sized cubed around 1cm X 1cm. Jicama is very resistant to becoming soft or overcooking, so small cubes are best here.
Heat a large pot of water over a high heat. Once boiling, add the cubed jicama and cook for anywhere between 15-30 minutes or until they reach your desired apple consistency. They don’t soften too much more in the oven, so it’s important to get the consistency right at this stage.
Once boiled, drain the jicama and return them to the pot. Add the butter, 60ml (1/4 cup) of the lemon juice, sugar, cinnamon and salt and stir to combine. Cook for 5 or so minutes or until a golden coloured syrup has formed in the pot.
Mix together the remaining lemon juice and corn starch to form a slurry. Pour the slurry over the jicama mixture and stir to combine. It should thicken quite quickly. Cook it for a minute or so, then remove it from the heat. Jicama don’t expel water as they cook, so the thickness of your ‘apple’ now will be the consistency once cooked. Add a splash of water if you prefer a saucier apple crumble. Set aside while you make the crumble.