In a non-metallic dish, combine the marrow, sugar, lemon juice and zest and soak for 24 hours in the fridge.
The next day, remove the marrow and lemon mixture from the fridge and slowly bring to the boil in a large saucepan or preserving pan. Tie the root ginger in a muslin bag and add to the pan.
Simmer for about 30 minutes until the marrow is soft, then add the stem ginger and citric acid. Test for the setting point by dropping a small amount of the jam onto a cold plate. If you push it with your finger and it wrinkles, it is ready.
Remove the muslin bag from the jam. Spoon the hot jam into sterilised jars leaving 1cm of head space, and seal.
Leave the jam completely undisturbed overnight until cold to ensure a good set.
For better flavour, try bruising the root ginger by tapping it with a mallet just tap it with a mallet before placing into the muslin bag.