Combine all the ingredients in a non-metallic bowl. Cover, then set aside to allow the flavours to mingle for 3 hours (at room temperature), or chill overnight.
Put the mixture in a large saucepan. bring to the boil, then leave to simmer for 7-10 minutes. To check if the jam is ready, spread a teaspoon of the mixture on a chilled plate and allow to cool slightly; the surface of the jam is at setting point if it wrinkles when pushed with your finger.
Divide the jam between three sterilised jars and seal while the mixture is still piping hot.
Tip Rhubarb and ginger make a great pairing but you could substitute the ginger for finely grated orange zest, a split vanilla pod or just make a simple plain rhubarb jam, if you like.
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