In a preserving pan, combine the apples, lemon juice and nutmeg. Pour in just enough water to cover the apples. Simmer gently for 90 minutes until the the apples have broken down, mashing to release the juice. Transfer the contents of the pan to a muslin bag. Suspend over a clean pan and allow to drip overnight.
The next day, measure the liquid in the clean pan and add 450g of sugar for every 600ml of liquid. Bring to the boil and begin testing for the setting point by placing a drop of the jelly onto a cold plate. If the jelly wrinkles when you push it with your finger, it is ready. Reaching the setting point may take up to 30 minutes.
Transfer into sterilised jam jars and seal immediately. Leave undisturbed in a cool, dark place for it to achieve a good set.