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Pare the zest from the lemons with a peeler, then juice them, keeping the juice, shells and any pips. Tie the shells and pips into a muslin bag. Put the marrow into a preserving pan with 2 tbsp of the lemon juice, then cook on a medium heat, stirring often, until the pieces are turning translucent and soft but not mushy. Bubble off any juices before stirring in the sugar, the rest of the juice, the zest, ginger and the muslin bag. Stir until the sugar dissolves.
Bring to the boil, then simmer for about 10-15 mins or until the marrow has softened completely and the jam has reached setting point (see Tips, below). Pot the jam into warm jars (see below). The flavour of the jam will mature and intensify over the next few months, so tuck it away in a dark, cool place.