Wash and drain the fruit, then cut the apples in half and add to a large, heavy-based saucepan with an equal quantity of water and the pared lemon zest. Bring to the boil, then reduce the heat and simmer for 20-25 mins until the apples are pulpy. Stir occasionally using the back of a wooden spoon to help break down the fruit.
Remove from the heat and allow to cool a little. Place the fruit pulp in a jelly bag and allow the juice to drip through into a large bowl (this will take several hours, so doing this overnight is ideal). If you force the fruit through, this will create a cloudy jelly.
Put two small, sterilised saucers in the freezer – you will use these to test the setting point of the jam. You can sterilise the saucers by washing them in hot soapy liquid, rinsing and placing in an oven at 180C/160C fan/gas 4 for 5 mins. Remove carefully and allow to cool.
Weigh the fruity liquid and return to a large saucepan. Weigh three quarters of sugar to your total liquid amount (for example, if you have 400g of liquid, add 300g of sugar).
Add the sugar to the juice with the lemon juice, and stir over a medium heat until completely dissolved. Turn up the heat and bring the fruit to the boil. Stir occasionally and allow to boil hard for about 8 mins, removing any scum with a spoon. If using a thermometer, you’re looking to reach 105c. If not, check the setting point of the liquid after 8 mins by removing a saucer from the freezer and adding a teaspoon of the liquid to the plate. Allow to sit for a minute, then push your finger through the liquid. If it starts to wrinkle, the jelly is ready. If not, return to the boil and try again after another minute.
Pour into sterilised jars and allow to cool completely. Store in a cool, dark place.