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Put a small plate in the freezer. Put the rhubarb into a preserving pan or your largest saucepan with the sugar and halved vanilla pods. Heat gently, stirring, until all the sugar has dissolved, then squeeze in the lemon juice and increase the heat.
Boil for about 10 mins, skimming off the scum as you go (the fruit should be soft). Test for setting point by spooning a little onto your chilled plate. After 1-2 mins, push your finger through the jam – if the surface wrinkles it is ready, if not, keep cooking for 2-min intervals, testing in between. (Or if you have a sugar thermometer it should reach 105C)
Once the jam is ready, let it cool for about 15 mins before ladling into warm sterilised jars and sealing. Will keep for 6 months in a cool, dark place.