Roughly tear 8 of the lime leaves and place in saucepan with apple, lemongrass, half of the chilli and 6 cups of water. Heat on high, until boiling, then return heat to low and simmer covered for one hour until the apples are very soft. Strain through a fine sieve or piece of muslin.
Measure apple liquid (you should have approximately 4 cups). Allow one cup of sugar for each cup of liquid.
Place apple liquid and sugar in a saucepan on low heat, stirring until the sugar dissolves. Increase heat to high. Boil uncovered, stirring occasionally, for 10-15 minutes until jelly sets when tested (see note).
Finely shred the remaining lime leaves and add to the jelly along with the remaining chilli. Simmer for 2 minutes.
Pour into hot sterilised jars and seal immediately.
Store the jelly in a cool, dark place for up to 3 months. Once opened, store in the fridge and use within 2 months.
*Note* to test if the jelly is ready, place a small saucer in the freezer before you start. Drop 1 teaspoon jelly onto the saucer to cool. If it's ready, the jelly should stay in place and form a skin on the surface.